Not every drive can be emptied before testing. If a USB stick or memory card already holds files you need to keep in place, FakeFlashTest’s Test Empty Space mode is the check designed for that situation. In this mode only the empty part of the drive gets exercised, while your existing files are left where they are. This guide explains how it works, what it can prove, and — equally important — what it cannot.
FakeFlashTest is a freeware, Windows-only, portable tool by Steve Si, available from the official source via our download page. For the complete workflow for a new drive, see our main USB capacity testing guide. For the destructive alternative, see the Quick Size Test guide — understanding both modes helps you choose the right one.
How Test Empty Space works
Instead of writing marker blocks across the whole reported capacity, Test Empty Space writes test files into the drive’s free space, filling that space progressively, and reads the files back to verify them. Crucially, it does not wait until the space is completely full before checking: it reads back and verifies at intervals as it goes — the developer’s implementation checks at points such as 3, 5, 9, and 17 GB. On a fake drive, that means corruption can be detected early, as soon as cumulative writes pass the real capacity, rather than only at the very end.
The design goal is preservation. Test files occupy space Windows reports as free, so files already stored on the drive are not deliberately overwritten. When the test finishes, the test files are removed and the drive’s contents should be as they were. That word — designed — matters, and the next section explains why a backup is still required.
Why you must still back up first
Test Empty Space is designed to preserve existing files, but always back up first. The reason is specific, not generic caution: if a file you already have sits in a bad area of the drive — for example, beyond the real capacity of a fake, where data has already wrapped around — that file can already be damaged, or can be further corrupted as the controller handles writes around it. The test cannot protect data that the drive’s own hacked firmware is mishandling.
So copy everything important off the drive, confirm the copies open, and only then test with the files still in place if you need to. If the backup succeeds and is verified, you also gain a better option: testing an emptied drive with the Quick Size Test, which samples the entire reported capacity rather than only the free remainder. Many careful users do exactly that — back up, empty, quick-test — and use Test Empty Space only when emptying genuinely is not possible.
The central limit: it only tests free space
Everything Test Empty Space can learn is confined to the space that is free at the moment you run it. If a drive is nearly full, there is almost no free space to fill, and the test can tell you almost nothing. A pass on a drive with very little free space is weak evidence — the unchecked, occupied portion could still hide the wrap-around point.
The practical fix is to free space deliberately before testing: move files to your computer (which you are doing for the backup anyway), empty the drive as far as you reasonably can, and then run the test so it has a large area to exercise. The more free space it can fill and read back, the more meaningful the verdict. If freeing space is impossible and the free remainder is tiny, treat any result as provisional and plan a full test on an emptied drive later.
There is a second limit shared with the Quick test. The developer is clear that FakeFlashTest is a quick screen: it does not thoroughly test all memory and does not check for every fault. Test Empty Space verifies the free space it writes, at the time it writes it. It is an excellent fake-capacity screen for a drive in use; it is not a complete certification of every cell on the device.
How to run it
Download FakeFlashTest via the download page, extract the official ZIP using the password fake, and Run as Administrator — without administrator rights, the tool lists no USB drives. Connect the drive directly, confirm its letter, select it carefully, choose Test Empty Space, and let it run to completion without removing the drive or letting the PC sleep.
Read the result in full. A failure during the read-back at any interval means the free-space writes could not all be stored correctly — exactly the signature of a fake or a failing device. If you encounter errors during this mode, our troubleshooting guide on Test Empty Space errors explains what they mean for files already on the drive and how to get a cleaner verdict on an emptied drive.
When to choose Quick instead
Choose Test Empty Space when the drive is in use, files must stay in place for now, and there is a substantial amount of free space to test. Choose the Quick Size Test when the drive is new, empty, or fully backed up and can be emptied — it samples the entire reported capacity, including the far end where fakes lie most obviously, and it gives the cleaner verdict.
Either way, act on a failure the same way: stop trusting the drive, preserve the result log as evidence, and do not promote a failed device to backup or dash-cam duty. A non-destructive check that tells you the truth early is valuable precisely because it lets you act while you still have choices.