Every tool has an origin story, and this one is unusually direct. RMPrepUSB is Steve Si’s multipurpose USB tool, and it contains a built-in feature called the Quick Size Test for exposing fake-capacity drives. FakeFlashTest is the standalone tool that grew out of that feature, focused on the single job of testing flash media for fake capacity. Comparing them is therefore less like comparing two competitors and more like comparing a dedicated tool with the workshop it came from: the standalone tool and the feature it grew from do the same core check, inside very different packages.
Both come from the same developer, which is why their test logic feels familiar. FakeFlashTest is freeware, Windows-only, and portable, with latest v1.1.5 described as the latest Beta, and it must be run as Administrator. RMPrepUSB is the larger, multipurpose program. One licensing distinction matters and is easy to get wrong: FakeFlashTest’s executable may be freely distributed under its freeware terms, while RMPrepUSB itself has different licensing and must not be redistributed. Keep that boundary straight if you ever share either tool with someone else.
The shared test at the centre
The Quick Size Test, in either home, writes marker data at intervals across the drive to expose the classic fake: hacked controller firmware reporting a small amount of real flash as a huge capacity. Once writes pass the real capacity, they wrap around and overwrite older data, so markers that should survive do not. The test is destructive in both tools. Nothing about running it inside RMPrepUSB makes it gentler, and nothing about the standalone tool makes it harsher; the check itself is the check. In RMPrepUSB’s case, the drive needs reformatting afterwards before it goes back into normal use, so plan for that step rather than being surprised by it.
FakeFlashTest adds a second mode its parent’s quick test does not centre on: Test Empty Space. That mode fills only the free space with test files and reads them back, with interval checks along the way, and it is designed to preserve existing files, although backing up first remains the sensible rule. That second mode is the clearest sign of FakeFlashTest’s purpose: it is not a general USB workshop, it is a testing tool, and its modes are arranged around the two situations testers actually face, an empty drive they can test destructively and a partly full drive they would rather not wipe.
| Tool | Method | Destructive? | Thoroughness | Platform | Cost |
|---|---|---|---|---|---|
| FakeFlashTest (Quick Size Test) | Marker blocks at intervals from the end of the drive | Yes | Quick screen; does not test all memory | Windows | Free (freeware, distributable) |
| RMPrepUSB Quick Size Test | Built-in quick size check inside a multipurpose USB tool | Yes; drive needs reformatting afterwards | Quick screen | Windows | Free; different licensing, do not redistribute |
Which one should you actually use?
For almost everyone reading this site, the answer is FakeFlashTest. It does one job, its two modes are labelled for the situations you will meet, and you can carry it as a small portable program without installing or learning a multipurpose suite. Our download page provides the current build, and how to test a USB flash drive for fake capacity walks through the standalone workflow from first launch to final result, including running as Administrator and reading the results correctly.
RMPrepUSB’s built-in test makes sense when you already use RMPrepUSB for its other USB work and the drive is already on your bench inside that workflow. Even then, remember the scope both tools share: a quick size test is a screen, not a full verification and not a fault check across all memory. The developer says so about FakeFlashTest, and the same honesty applies to the parent feature. If a drive passes the quick test and will hold important data, graduate to a full write-and-verify test such as H2testw, as laid out in FakeFlashTest vs H2testw, before you trust it.
After the test, whichever tool you used
A fail means the same thing in both programs: the drive is not the capacity it claims, data written past the real limit has likely been overwritten and is usually unrecoverable, and the drive should not be used for anything that matters. Follow what to do next when a fake drive is confirmed rather than reformatting and hoping. A pass, equally, means the same modest thing in both: the sampled check found no lie. Treat passes as permission to test further, not as proof. Our hub of the best fake USB drive test tools and the guide for testing microSD cards show the full ladder from screen to proof.
One habit whichever version you run
Both versions reward the same discipline: test on arrival, before the drive earns a job. A quick size check is at its most valuable in the first days you own a drive, while the return window is open and the drive is still empty, when a destructive test costs you nothing and a fail costs the seller, not you. Test every bargain drive this way, even from sellers who were honest last time, because marketplace stock changes without notice. Keep the result until the return window closes, and only then promote the drive to ordinary use, still with backups for anything that matters. That arrival ritual, more than any single program, is what stops fake capacity from ever reaching your real files.
Verdict
FakeFlashTest is the tool to download for fake-capacity testing: focused, portable, free, and built for exactly this job. RMPrepUSB’s Quick Size Test is its respected parent feature, worth using if RMPrepUSB is already your workshop, with reformatting afterwards and different licensing to respect. Same developer, same core idea, different scope. For a single-purpose test with the clearest workflow, start with the FakeFlashTest download, and keep the parent tool in mind as the broader suite it was always meant to be.