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:

1. 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.

2. 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 NTD 100 (32 GB) to NTD 500 (128 GB).

   Examples include:

   • Kingston DT Max 256G 256 GB: NTD 799
   • ADATA UE800 256G: NTD 999
   • Silicon Power Marvel M80 250G: NTD 945
   • Transcend ESD310C 256GB: NTD 1199

3. 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

Regardless of the internal protocol or technology, external drives communicate with the host via USB.

There are many USB versions and names, often confusing and perplexing.

Here are the three most common USB versions:

1. USB 3.2 Gen 1: Maximum speed of 500 MB/s.
2. USB 3.2 Gen 2: Maximum speed of 1200 MB/s.
3. USB 3.2 Gen 2x2: Maximum speed of 2400 MB/s.

Detailed comparison of USB names, speeds, and cable lengths is provided in the table below:

For example, the iPhone 15 Pro supports USB 3.2 Gen 2 speeds, reaching up to 1200 MB/s, while the iPhone 15 only supports USB 2.0, with a maximum speed of 480 Mbps (60 MB/s).

When converting an NVMe M.2 SSD into an external drive, the mainstream SSDs in 2024 are USB 3.2 Gen 2, with actual read/write speeds around 1000 Mbps.

Therefore, when choosing an NVMe M.2 SSD, ensure it has a read/write speed above 1000 MB/s.

For instance, “KIOXIA Exceria G2 1TB/M.2 PCIe Gen3/Read:2100M/Write:1700M/TLC/5 years warranty” indicates sequential read/write speeds of 2100 MB/s and 1700 MB/s, using TLC chips, with a capacity of 1 TB.

The theoretical maximum transfer speed may vary slightly depending on the USB coding method. Actual speeds depend on different chips and caching mechanisms; consult online performance benchmarks for more details. The actual single file speed refers to the read speed, and the actual single file write speed is usually similar or lower.

NVMe M.2 SSD Purchase Considerations

TLC or QLC?

Prioritize TLC over QLC; QLC is not the mainstream choice currently.

When selecting SSDs, you often see TLC (3) and QLC (4), occasionally MLC (2).

The numbers in parentheses indicate the number of bits each cell can store. Smaller numbers mean more cells are required to achieve the same capacity, offering more redundancy to prevent read/write failures, equating to longer lifespan and faster data processing.

PCIe Gen 3 or PCIe Gen 4?

Most external enclosures use PCIe Gen 3 controller chips, capable of reaching USB 3.2 Gen 2 speeds. Thus, PCIe Gen 3 is sufficient, with read/write speeds exceeding 1200 MB/s. Choosing PCIe Gen 4 is also possible as it is backward compatible with PCIe Gen 3.

If opting for USB 3.2 Gen 2×2, ensure the external enclosure’s controller chip supports it, and the NVMe M.2 SSD has read/write speeds exceeding 2400 MB/s.

Is DRAM Necessary?

Choose M.2 SSDs with at least 1 GB of DRAM; avoid DRAMless options.

DRAM (Dynamic Random Access Memory) in NVMe M.2 SSDs acts as a cache, speeding up data transfer, especially during random read/write operations. This is because DRAM can store the file mapping table, allowing the SSD to quickly locate the data’s actual position.

Some NVMe M.2 SSDs do not use additional DRAM chips to save costs. These NVMe SSDs are labeled DRAMless and rely on the NAND flash memory or the host memory (computer’s DRAM) for mapping and caching data.

Reading from the computer’s DRAM requires Host Memory Buffer (HMB) technology, which accesses the computer’s DRAM via the PCIe interface. However, HMB technology is not supported by USB protocols for external drives.

Choosing External Enclosure Controller Chips?

When selecting external enclosure controller chips, consider the protocol supported by the M.2 slot. The market offers SATA and PCIe (NVMe) options. The controller chip bridges PCIe or SATA to USB, directly affecting the USB 3.2 speed supported by the enclosure. Common chips include:

These chips are manufactured in Taiwan, even SSD enclosures bought on Taobao use one of these chips. There are not many performance tests available for these chips, so they are not a factor in SSD performance differences. Additionally, for USB 3.2 Gen 2x2, the SSD’s read/write speed should exceed 2400 MB/s to fully utilize the speed.

You may also encounter the older Intel JHL7440 chip, which can be replaced by the ASM2464PD chip.

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:

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

Conclusion

Considering actual application, capacity, cost, thermal management, and brand reputation, the final choice of SSD often depends on multiple factors. Before making a selection, research online performance benchmarks.

If you lack the time or energy to evaluate different M.2 SSDs, choosing well-known brands and popular pre-built SSDs is less risky.

For instance, common external SSD products in 2024 include:

1. ADATA SE900G
2. SanDisk Extreme PRO Portable SSD V2
3. SK hynix Beetle X31
4. ADATA SD810
5. Kingston XS2000 Portable SSD
6. Crucial X10 Pro

These external SSDs are commendable in terms of design and protection. However, due to size constraints, thermal design might be compromised.

Despite all supporting USB 3.2 Gen 2×2, some products may achieve maximum speeds, while others may not due to different NVMe controllers or caching settings.

When purchasing, refer to other users’ reviews. However, these high-end products generally come with a higher price tag.

References

1. 兩種 M.2 SSD 固態硬碟的類型：SATA 和 NVMe
2. NVMe vs M.2 vs SATA vs PCIe: What are these SSDs?
3. 最新SSD固態硬碟顆粒SLC/MLC/TLC/QLC有什麼區別？
4. USB 3.1 Gen 1、Gen 2 及 USB 3.2 的差異為何？
5. The New USB standard: USB 3.2 Explained
6. USB4® | USB-IF
7. 使用 iPhone 15 的 USB-C 連接器充電及連接
8. 原價屋查價
9. 欣亞數位線上估價
10. SSD Database
11. SSD_Buying_Guide_List.pdf
12. Understanding File Systems