Buying a USB flash drive should be simple, but fake-capacity drives have made a quick check essential. This flagship step-by-step USB capacity check shows you how to test a drive with FakeFlashTest, a freeware Windows tool by Steve Si, the developer behind RMPrepUSB. FakeFlashTest is Windows-only, portable with no install, and freeware — it is not open source. Used correctly, it can tell you whether the capacity Windows reports is the capacity you actually have, before you trust the drive with photos, work files, or backups.
Start from the right place: get the tool only via our download page, which links to the official password-protected ZIP from rmprepusb.com. Do not download copies from random file-hosting sites. You will also want this guide open alongside the tool, because the most common mistakes happen before the test even starts — choosing the wrong drive letter, or testing a drive that still holds the only copy of important files.
Why fake capacity matters
A fake drive is not just a slow or faulty drive. Its controller firmware has been hacked — often described as clocked — so that a small flash chip, for example 8 GB or 16 GB of real storage, reports itself to Windows as 512 GB, 1 TB, 2 TB, or even more. Everything looks normal at first. You can copy files, see folders, and check properties, and Windows will happily show the inflated size.
The problem appears when you write past the real capacity. At that point the drive wraps around and silently overwrites data you saved earlier. The folder listing can still look normal while the files inside are corrupt. Overwritten data is usually not recoverable. That is why testing on arrival, while you can still return the drive, matters far more than testing months later after you have filled it.
Step 1: Back up anything on the drive first
Before any test, copy anything you care about off the drive to your computer or another storage device, and check that the copies open. This is not optional caution. One of FakeFlashTest’s two modes is destructive by design, and even the gentler mode can expose corruption in files that happen to sit in bad areas of a fake drive.
If the drive is brand new and empty, this step is quick. If you have already used it, treat the backup as your safety net. A test result is useful; a test that costs you your only copy of a file is not. Back up first, every time.
Step 2: Download and extract FakeFlashTest correctly
Go to the download page and follow the official link. The official download for the latest version, v1.1.5 — described by the developer as the latest Beta build — is a password-protected ZIP. The extraction password is fake. The password is not a trick: it was set to reduce false blocks by Windows Defender, which can be suspicious of low-level storage tools.
Extract the ZIP to a folder on your computer, not onto the drive you are about to test. Keep the extracted program somewhere easy to find, such as your Downloads folder or Desktop. Because FakeFlashTest is portable, there is nothing to install and nothing to uninstall afterwards. If Windows asks questions about an unfamiliar program, pause and check that you really did download it from the developer’s site via the link above — source matters more than dismissing warnings.
Step 3: Run as administrator and pick the right drive
Right-click the FakeFlashTest program and choose Run as Administrator. This step is required: the developer states clearly that the tool must run as administrator or no USB drives will be listed. If the drive list is empty, running without administrator rights is the first thing to check, before you assume anything is broken.
Now choose the drive letter with great care. Check the letter in File Explorer and in Disk Management if you are unsure, and match the size and label to the drive you inserted. The Quick Size Test destroys data on the selected drive, so selecting your system drive or a backup drive by mistake would be a serious error. Take ten seconds here. Remove other USB storage you do not need for the test if that helps you identify the target with confidence.
Step 4: Run the Quick Size Test first
For an empty or backed-up drive, start with the Quick Size Test. As explained in detail in our Quick Size Test guide, this mode writes and reads back 512-byte marker blocks at intervals across the drive, starting from the end. Because it samples across the whole reported capacity instead of filling every byte, it is designed to be fast.
The developer’s own example diagnosed a fake 16 GB drive in 27 seconds. That is a single example, not a promise. A large or slow drive can take much longer, and you should let the test finish rather than judging it in the first minute. Remember what this test is and is not: the developer is explicit that FakeFlashTest is a quick screen. It does not thoroughly test all memory and does not check for every fault. A pass is reassuring, not a lifetime guarantee.
Step 5: Consider Test Empty Space or a full test
If the drive holds files you cannot move, the alternative is Test Empty Space, covered in our Test Empty Space guide. It writes test files only into the free space and reads them back, checking at intervals, and it is designed to preserve existing files. It still only tests the free space, so a nearly full drive gives it very little to work with.
For the most thorough check, a full write-and-verify tool such as H2testw remains the reference. It fills the entire drive and verifies every byte, which is more complete than any sampling test but can take hours. Many people use FakeFlashTest first as a fast screen, then run a full test overnight if the drive passes and they plan to trust it with important data. That two-stage approach is sensible and honest about each tool’s limits.
Step 6: Read the result and decide what to do next
When the test finishes, read the log carefully rather than looking only for the word pass or fail. Our guide on how to read FakeFlashTest results walks through what a pass means, what a failure tells you about real capacity, and why you should screenshot the log. If the drive fails, the log becomes your evidence.
If it is confirmed fake, follow the steps in Fake Drive Confirmed: What to Do Next: stop writing to it, salvage what still opens, pursue a refund, and replace it with a genuine drive from a reputable seller. If it passes the quick screen, format it, use it normally, and still keep backups elsewhere. No single test — quick or full — replaces keeping copies of files you cannot afford to lose.
That is the whole routine: back up, download from the official source, run as administrator, choose the letter carefully, test, and act on the result while returns are still possible. Five careful minutes on arrival can save you from discovering a fake the hard way, months later, when the files it overwrote are the ones you needed most.