Portable SSD Speeds Are Real for About 84GB: The Specs Nobody Prints on the Box

The advertised speed lasts about 84 gigabytes

Tom’s Hardware measured a 2TB Samsung T7 writing at 867 MB/s — close to its 1,000 MB/s rating — for roughly 84GB. Then it dropped to 337 MB/s and stayed there for the rest of the transfer. Nothing failed. That is how the drive is built to behave, and it is the most important thing to understand before you spend money on portable storage. Every “up to 1,050 MB/s” figure on a box is a burst number describing the first slice of a transfer, not the transfer.

Why it happens: pseudo-SLC caching

TLC flash stores three bits per cell and QLC stores four. Packing multiple bits into a cell is slow, so controllers reserve a region of that same flash and run it in fast single-bit mode. Incoming data lands in this pseudo-SLC cache first and is folded into dense native mode later, while the drive is idle.

The cache is finite, scales with capacity, and shrinks as the drive fills. Once exhausted you write at the flash’s native speed — commonly 400 to 1,500 MB/s on TLC portable drives, lower on QLC. Measured examples:

• Samsung T7 (2TB): 867 MB/s for ~84GB, then 337 MB/s (Tom’s Hardware).
• Crucial X10 (2TB): around 985 MB/s until roughly 315GB written, then about 800 MB/s (ThePCEnthusiast) — a far gentler cliff.
• StorageReview’s six-hour sequential write test: a Seagate FireCuda Portable held just over 1.9 GB/s for about 45 minutes before settling near 1.54 GB/s; a Samsung T7 sat around 240 MB/s in steady state; the SanDisk Extreme Pro held 1.79 GB/s.
• Samsung T9: advertises a TurboWrite buffer of up to 180GB.

The rule: if your largest single transfer is under about 50GB, the headline number is roughly what you get. If you routinely dump 200GB card offloads, the post-cache figure is your real speed — and almost nobody publishes it. Samsung’s T7 Shield is the exception, rated above 900 MB/s across the whole drive, which makes it a better offload drive than the plain T7 despite near-identical headline specs.

Check Samsung T7 Shield on Amazon

DRAM vs DRAM-less: mostly overblown, with one real exception

A DRAM cache holds the flash translation layer map. Drives without it keep a tiny map in on-controller SRAM or borrow system RAM through Host Memory Buffer (HMB). Forum consensus treats DRAM-less as disqualifying; for sequential media work it usually is not, because copying video is dominated by cache size and NAND speed, not map lookups.

The exception is decision-changing. HMB requires direct NVMe access to host memory, and a standard USB-to-NVMe bridge does not provide it — so a DRAM-less SSD in an ordinary USB enclosure loses HMB entirely and falls back to its small internal SRAM. USB4 and Thunderbolt enclosures that tunnel PCIe can preserve it. If you are building a drive from a bare NVMe stick, that is where DRAM-less actually costs you.

The interface reality: 10, 20, and 40 gigabits

Gen 2x2 reaches 20Gbps by running two 10Gbps lanes in parallel, and host support for it is optional. Apple never implemented it. No Mac supports USB 3.2 Gen 2x2 — not Apple Silicon, not machines with Thunderbolt 5 ports. Plug a Gen 2x2 drive like the Samsung T9 or Crucial X10 Pro into a Mac and it negotiates down to 10Gbps, roughly 1.0 GB/s. Howard Oakley found Intel Macs on macOS 14 or later can reach the full 20Gbps through a Kensington SD5000T5 EQ Thunderbolt 5 dock, around 1.5–1.8 GB/s, but that does not work on Apple Silicon. For Mac users the Gen 2x2 premium buys nothing.

Check Crucial X9 on Amazon

Heat: the second cliff

NVMe drives typically idle around 45°C and reach 70–80°C under sustained load, with throttling generally starting in the 80s. Reviewers have recorded a drive hitting 86°C and collapsing to roughly 100 MB/s, and a bare NVMe stick averaging 1,027 MB/s over fifteen minutes where the same drive with a heatsink averaged 3,681 MB/s.

Retail portable SSDs behave better than bare sticks in plastic because the aluminium body is the heatsink. Testers copying a 500GB file to a Samsung T9 recorded a 64°C internal maximum, a 45°C shell, and no measurable throttling even across transfers up to 1.2TB. Treat any bus-powered 3 GB/s USB4 drive with suspicion until you see a long-run test.

The SanDisk Extreme problem

Any portable SSD guide that skips this is doing you a disservice.

From 2023, users of the SanDisk Extreme Portable V2 (SDSSDE61) 4TB, the SanDisk Extreme Pro Portable V2 (SDSSDE81) in 1TB, 2TB and 4TB, and the 4TB WD My Passport SSD (WDBAGF0040BGY) reported drives disconnecting and becoming unreadable, taking the data with them. Units manufactured after November 2022 appear most affected, with 4TB variants failing most often.

Western Digital shipped a firmware update in May 2023. Data-recovery firms concluded the root cause was physical — oversized components with weak PCB contact, and voids in solder joints that crack as the drive heats and cools during long transfers. The firmware changed disconnection handling; it did not fix solder. Failures continued afterwards, including on drives belonging to reviewers at The Verge and Ars Technica. Later revisions reportedly added epoxy reinforcement, and recovery firms say those have still failed.

Krum v. Western Digital (No. 5:23-cv-04152, N.D. Cal.) was filed in August 2023. In June 2024 Judge Rita Lin partially denied Western Digital’s motion to dismiss, letting the fraud-by-omission and nationwide class claims proceed and rejecting the argument that the firmware had cured the defect. The case remains in discovery. There has been no settlement and no recall.

The Extreme line often looks like the value pick. Given documented, unresolved data loss with no manufacturer remedy, we do not recommend it as a primary or sole copy of anything.

The Japanese angle: Buffalo and I-O Data

Walk into a Bic Camera and the shelf looks nothing like Best Buy’s — Buffalo and I-O Data dominate, with model numbers instead of product names.

Buffalo’s SSD-PGM series (the SSD-PGM1.9U3-B, for example) is USB 3.2 Gen 2 Type-C in 240GB, 480GB, 960GB and 1.9TB, 50 × 10.9 × 95 mm, about 52g. The differentiators are software: AES-256 SecureLock Mobile2 encryption and “Mimamori Aizu,” which flags a dying drive via an LED. The SSD-SDH series adds a dual Type-C/Type-A connector and a MIL shock rating. I-O Data runs from the SSPG-USCB and SSPB-USC at roughly 500–530 MB/s up to the SSPF-USC at 1,000 MB/s.

Does missing out matter? Not for speed. Buffalo Americas sells in the US — the SSD-PUT stick and SSD-PG series, up to about 600 MB/s — a slower, narrower lineup, none of which beats a T7 Shield. I-O Data is effectively absent from US retail. What you would import is the health-monitoring and encryption software: nice, and not worth the premium.

Check Buffalo SSD-PGM on Amazon Japan

Practical advice

Capacity. Buy larger than you need: cache scales with capacity and shrinks as the drive fills, so a half-empty 2TB drive sustains a big transfer better than a nearly-full 1TB one. Buffalo’s odd 960GB and 1.9TB labels are honest decimal capacities after overprovisioning.

Enclosure plus bare NVMe. A metal USB4 enclosure with proper thermal pads beats most retail drives on long transfers, but you lose bundled encryption and a single warranty, and a DRAM-less stick behind a standard bridge loses HMB. Buy DRAM-equipped flash, or an enclosure that tunnels PCIe.

Timing. NAND contract prices rose sharply through 2026 as manufacturers shifted capacity toward AI memory, roughly doubling consumer SSD prices since late 2025. Samsung’s T9 carries MSRPs of $139.99 (1TB), $239.99 (2TB) and $439.99 (4TB), but August 2026 street listings sat well above — a T7 Shield 2TB at $428.99 from a third-party seller, 4TB at $929.99. If you can wait, wait.

Encryption. Samsung’s T7, T7 Shield and T9 offer hardware AES-256 via Samsung’s software and a password; Buffalo’s SecureLock Mobile2 does the same in Japan. Otherwise BitLocker To Go, FileVault on an APFS volume, or VeraCrypt will encrypt any drive — less convenient, but they outlive the manufacturer.

Check USB4 NVMe enclosures on Amazon

Summary

Ignore the number on the box and ask three questions. How big is the pseudo-SLC cache and what happens after it fills — for large offloads that post-cache figure is the only speed that matters. Does the interface work on your machine, because Gen 2x2 is dead weight on every Mac ever made. And can the enclosure shed heat over a twenty-minute transfer, not a twenty-second benchmark.

For most people a 10Gbps drive with honest sustained write, like the T7 Shield, beats a faster-on-paper Gen 2x2 drive. Skip the SanDisk Extreme line until the data-loss issue has a real remedy. And whatever you buy, the drive is a copy, never the copy.

The cable matters more than people think — see USB-C cables explained, and browse the rest of our English reviews.