M.2 SSD External Drive Buying Guide

Introduction

How should you go about purchasing an external solid-state drive (external SSD) today?

Additionally, there are also USB flash drives and traditional external hard drives (external HDD) on the market.

Their characteristics and differences are summarized in the table below:

Features	USB Flash Drive	Solid State Drive (SSD)	External (Mechanical) Hard Drive (HDD)
Speed	Slow	Very fast, low read/write latency	Slowest, high read/write latency
Capacity	8 GB to 1 TB	250, 500 GB, 1, 2, 4 TB	1, 2, 4, 5, 6, 8, 10 TB
Size	Compact	Small, portable	Larger, relatively heavier
Thermal Management	Poor	Heat sinks	No special treatment needed
Advantages	Cheap, portable	Fast speed, compact size	Large capacity, low cost per GB
Disadvantages	Speed varies	Generally more expensive, not for long-term power usage	High power consumption, prone to physical damage, noisy
Market average cost per GB (TWD/GB)	≈ 3.58	External ≈ 2.59 or DIY M.2 ≈ 2.33~2.54	≈ 1.26

External Mechanical Hard Drives (HDD): Known for their high capacity and low cost per unit, but slower speeds and higher susceptibility to physical damage due to their mechanical structure. Often preferred for NAS assembly.
USB Flash Drives:
The main differences between USB flash drives and solid-state drives lie in capacity and speed. Flash drives are compact and portable, but their speed can vary significantly due to cost-saving measures.
USB flash drives have the highest cost per unit but are commonly available in capacities of 32, 64, and 128 GB, with prices ranging from TWD 100 (32 GB) to TWD 500 (128 GB).
Examples include:
- Kingston DT Max 256 GB: TWD 799
- ADATA UE800 256 GB: TWD 999
- Silicon Power Marvel M80 250 GB: TWD 945
- Transcend ESD310C 256 GB: TWD 1,199

SSD:

The SSD market is divided into ‘pre-built SSDs’ and ‘DIY NVMe M.2 SSD + external enclosure.’

Pre-built SSDs	DIY NVMe M.2 SSD + External Enclosure
Compact, comes with an SSD and a cable, ready to use out of the box.	Flexible, allows choosing different brands, memory chips, controllers, and can be cheaper overall.

M.2 refers to a small, efficient interface standard that supports multiple signal protocols. For SSDs, the protocols are SATA and PCI Express (PCIe).
SATA: Maximum speed of 6 Gbps, with a practical limit of 600 MB/s. Prefer NVMe over SATA to fully utilize USB 3.2 Gen 2’s 1000 MB/s.
NVMe: A protocol designed for PCIe-based flash memory, marked as PCIe Gen3 or PCIe Gen4 during SSD selection. PCIe Gen 4 offers faster speeds, but for external SSDs using USB Gen 2, PCIe Gen 3 is sufficient, achieving around 1000 MB/s.

This article focuses on choosing NVMe M.2 SSDs + external enclosures, highlighting the key parameters to consider.

Priority Order for Drive Selection

Portability: Flash Drive > SSD > HDD
Performance: SSD > Flash Drive > HDD
Large Capacity Consideration: HDD > SSD > Flash Drive
Cost: Flash Drive > HDD > SSD
OS Boot: SSD > HDD > Flash Drive
Durability: Flash Drive > SSD » HDD
Power Consumption: SSD > Flash Drive > HDD
If Money is No Object: SSD > Flash Drive > HDD

USB External Interface Specifications

Regardless of the protocol or technology used internally in an external hard drive, communication with the host or other devices is ultimately conducted via the USB interface. The USB specifications have undergone several revisions, and the naming and specifications of each version can be confusing. The commonly used USB versions at present include:

USB 3.2 Gen 1: Maximum transfer speed of approximately 500 MB/s.
USB 3.2 Gen 2: Maximum transfer speed of approximately 1200 MB/s.
USB 3.2 Gen 2×2: Maximum transfer speed of approximately 2400 MB/s.

The table below lists the marketing names, corresponding specifications, transfer bandwidth, theoretical and actual speeds, previous naming, interface options, and cable lengths for each USB specification:

Marketing Name	Corresponding Specification	Transfer Bandwidth	Theoretical Maximum Speed	Actual Single-File Read Speed	Previously Known As	Interface Options	Cable Length
USB 5Gbps	USB 3.2 Gen 1	5 Gbps	500 MB/s	450-500 MB/s	USB 3.1 Gen 1, USB 3.0	USB-A, USB Micro B, USB-C	< 1 m
USB 10Gbps	USB 3.2 Gen 2	10 Gbps	≈ 1,200 MB/s	950-1,000 MB/s	USB 3.1 Gen 2, USB 3.1	USB-A, USB Micro B, USB-C	< 1 m
USB 20Gbps	USB 3.2 Gen 2×2	20 Gbps	≈ 2,400 MB/s	1800-1,900 MB/s	—	USB-C	< 0.8 m
USB 20Gbps	USB4 Gen 2×2	20 Gbps	≈ 2,400 MB/s	1800-1,900 MB/s	—	USB-C	< 0.8 m
USB 40Gbps	USB4 Gen 3×2	40 Gbps	≈ 4,800 MB/s	3,700-3,800 MB/s	—	USB-C	< 0.8 m
USB 80Gbps	USB4 v2.0	80 Gbps	≈ 9,600 MB/s	5,000-6,250 MB/s	—	USB-C	< 0.8 m

The iPhone 15 Pro supports the USB 10Gbps (USB 3.2 Gen 2) specification, with a theoretical maximum speed of 1200 MB/s; whereas the iPhone 15 only supports USB 2.0, with a maximum speed of 480 Mbps (approximately 60 MB/s).

If an NVMe M.2 SSD is used as an external hard drive, mainstream products in 2024 all adopt the USB 10Gbps specification, with actual read/write speeds of approximately 1000 MB/s. When selecting an NVMe M.2 SSD, it is recommended to choose products with read/write speeds exceeding 1000 MB/s. For example, the “Armor KIOXIA Exceria G2 1TB/M.2 PCIe Gen3” product can achieve sequential read and write speeds of 2100 MB/s and 1700 MB/s respectively, uses TLC NAND, has a capacity of 1 TB, and comes with a five-year warranty.

Due to differences in USB encoding methods, the theoretical maximum transfer speed may vary slightly. Actual speeds differ depending on the type of NAND flash and caching mechanisms; please refer to online performance benchmark tests. The actual single-file speed refers only to the read speed, and the actual single-file write speed is usually the same or slightly lower.
Currently, devices supporting USB4 v2.0 in the market are mainly based on Intel Thunderbolt 5 chips. These chips conform to the USB4 v2.0 specifications and are compatible with it; however, the USB4 v2.0 certification program is still in the planning stage by USB-IF, and there are no officially certified products yet.
In the USB 3.x specifications, the naming includes a space; whereas the official name for USB4 removes the space and does not include the “.0” suffix.

NVMe M.2 SSD Purchase Considerations

TLC or QLC?

When selecting an SSD (Solid-State Drive), TLC (Triple-Level Cell) is generally the preferred option over QLC (Quad-Level Cell). QLC is not the mainstream choice in the current market, especially for applications requiring high durability and stability.

Common types of SSDs include TLC (storing 3 bits per cell), QLC (storing 4 bits per cell), and occasionally MLC (Multi-Level Cell, storing 2 bits per cell).

Relationship between Bit Count and Number of Cells: The fewer bits stored per cell, the more cells are required to achieve the same capacity. For example, MLC (2 bits) requires more cells than TLC (3 bits) to reach the same storage capacity.
Durability and Lifespan: With more cells available, data can have greater redundancy to prevent read/write failures, resulting in a longer lifespan. Since MLC stores fewer bits per cell, it generally has higher durability due to the greater number of cells used for the same capacity.
Data Processing Speed: Moreover, processing cells with fewer bits is relatively faster during data read/write operations.

Therefore, when choosing an SSD, particularly for high-performance and long-lasting applications, TLC is a more suitable choice than QLC.

PCIe Gen 3 or PCIe Gen 4?

Most external enclosures on the market are equipped with a controller chip that supports PCIe Gen 3, paired with USB 3.2 Gen 2 transfer speeds (theoretical value of approximately 10 Gbps, with actual transfer speeds around 1000–1200 MB/s). Thus, for general use, selecting PCIe Gen 3 to ensure bandwidth read/write speeds exceeding 1200 MB/s is sufficient.

For assembling advanced USB 3.2 Gen 2×2 systems to achieve faster transfer speeds, it is essential to verify that the enclosure’s controller chip supports this standard and that the chosen NVMe M.2 SSD has read/write performance greater than 2400 MB/s.

Is DRAM Necessary?

When selecting an NVMe M.2 SSD, it is recommended to choose models with at least 1 GB of DRAM, avoiding DRAMless versions.

DRAM (Dynamic Random Access Memory) acts as a cache in NVMe M.2 SSDs, significantly enhancing data transfer efficiency, especially when handling random read/write operations. Its primary function is to store the mapping table that records the actual storage locations of data in the NAND flash memory. With DRAM, the SSD can quickly locate and access data, considerably reducing response times and improving overall performance.

To reduce costs, some NVMe M.2 SSDs do not include an additional DRAM chip; these products are usually labeled as “DRAMless”. SSDs without independent DRAM rely on two alternative methods to store the mapping table and cache data:

NAND Flash Memory Itself: Directly storing the mapping table in NAND, though NAND’s access speed is significantly lower than that of DRAM, leading to performance degradation.
Host Memory (the computer’s DRAM): Through Host Memory Buffer (HMB) technology, the SSD can directly access the computer’s DRAM via the PCIe interface as a cache.

Since external hard drives use the USB protocol, and HMB technology is only applicable to the PCIe interface—not the USB protocol—DRAMless SSDs in USB external applications cannot utilize host memory to boost performance.

Choosing External Enclosure Controller Chips?

When selecting an external hard drive enclosure controller chip, the primary consideration is the protocol supported by the M.2 slot, which can be either SATA or PCIe (NVMe). The controller chip is responsible for bridging PCIe or SATA and converting it to a USB interface. This directly affects the USB 3.2 speed supported by the external hard drive enclosure. Common chips on the market include:

Controller Chip	USB Protocol	Transfer Bandwidth
Realtek RTL9210	USB 3.2 Gen 2	10 Gbps
Realtek RTL9210b	USB 3.2 Gen 2 + SATA	10 Gbps
JMS583	USB 3.2 Gen 2	10 Gbps
ASM2362	USB 3.2 Gen 2	10 Gbps
ASM2364	USB 3.2 Gen 2×2	20 Gbps
ASM2464PD	USB 4/Thunderbolt Gen 3×2	40 Gbps

All the aforementioned chips are manufactured in Taiwan; even SSD external enclosures purchased on Taobao typically utilize one of these three chips. There are few tests online for these three controller chips, so the differences in SSD performance are not considered a significant factor. In addition, for USB 3.2 Gen 2×2, the SSD’s read/write speeds must exceed 2400 MB/s to fully utilize its potential.

Some products may use the Intel JHL7440 chip, which is relatively outdated by today’s standards. In such cases, the ASM2464PD chip can be chosen as a substitute.

Sliding Cover or Clamshell Enclosure?

Prefer clamshell over sliding cover enclosures.

Many enclosures marketed as tool-free are sliding cover types, where the M.2 SSD can be removed like a drawer. Sliding cover enclosures often have independent heat sinks that do not effectively conduct heat to the case. Conversely, clamshell enclosures do not slide, allowing direct heat transfer from the heat sink to the case. More reviews can be found online for informed choices.

Normal Operating Temperature for SSDs?

For desktop use, SSD temperatures are typically around 42°C, with normal operating temperatures between 50-55°C. Exceeding 60°C can reduce speed and lifespan.

Formatting Choices for Drives?

exFAT is the preferred choice for cross-device compatibility.

Choosing exFAT allows for data transfer across devices (Windows, macOS, iOS, Android) with seamless reading and writing.

For specific device use, choose the corresponding format:

Windows: NTFS
macOS / iPad / iOS: AFPS
Android: exFAT

Not Achieving Expected Speeds?

Check cable length, USB port support, controller chip support for PCIe, controller chip firmware, and SSD cache.

Achieving expected speeds is crucial when assembling or purchasing high-speed SSDs. If the speed is not as expected, it can be frustrating.

To troubleshoot speed issues, check:

Is the cable shorter than 0.8 m?
Does the computer’s USB port support the required USB version?
Is the Type C to Type A adapter faulty?
Does the M.2 SSD’s read/write speed meet USB’s speed (PCIe read/write only 2000 Mbps cannot reach USB4 speeds)?
Performance of the controller chip
Is the controller chip firmware up-to-date?
Is the SSD cache full, causing high temperatures and reduced speeds?
Is write caching enabled?

Conclusion

Considering factors such as practical application, capacity, cost, heat dissipation, and brand reputation, the decision on which SSD to choose often depends on these aspects. It is advisable to review online benchmarks for read and write speeds before making a choice.

If you are not inclined to evaluate which M.2 SSD to select, opting for a well-known, widely purchased SSD from major manufacturers is less likely to lead to disappointment.

For example, the external SSD products commonly seen in the market in 2025 include:

ADATA SD810
ADATA SE880
ADATA SE900G
Crucial X9 Pro
Crucial X10 Pro
Kingston XS1000
Kingston XS2000
SanDisk Extreme PRO Portable SSD V2
SK hynix Beetle X31
TeamGroup T-Force M200

These external SSDs excel in terms of design and protection. However, due to their compact size, there may be compromises in thermal management.

Additionally, although they all support USB 3.2 Gen 2×2 (20 Gbps), differences in NVMe controller chips and cache configurations mean that some products can achieve their maximum speeds, while others may perform slightly differently.

It is recommended to consult real-world performance reviews before purchasing to ensure the product meets your needs. The only certainty is that the prices for these end products are relatively high.