How to Use USBFillSpace to Test Flash Drive Capacity

USBFillSpace is an older, niche Windows utility originally developed by SoftSet to completely saturate a flash drive’s remaining storage capacity. How It Works and What It Actually Does

While many users look to USBFillSpace to test for fake or counterfeit high-capacity drives, it is not a true diagnostics testing tool.

The Original Intent: USBFillSpace was primarily designed as a security utility. In the early 2010s, viruses frequently spread via autorun.inf files on removable drives. USBFillSpace allowed users to deliberately fill 100% of a drive’s remaining blocks with dummy data. This blocked incoming malware from writing itself to the drive.

The “Capacity Test” Function: By writing dummy files until the operating system throws a “disk full” error, you can see exactly how many gigabytes of data the drive accepted before stopping. Why USBFillSpace is Outdated for Fake Drive Testing

Counterfeit USB drives use modified firmware to trick your computer into displaying an inflated capacity (e.g., a 32 GB drive spoofed to show 2 TB). When you write data past the physical limit (32 GB) on a spoofed drive, it will keep accepting data by secretly looping back and overwriting your old files, all while claiming the write was successful.

Because USBFillSpace only writes files and does not verify them, a fake drive will pass a USBFillSpace test without throwing an error, leaving you with corrupted, unreadable files. The Modern, Reliable Alternatives

Fake USB Drives & SD Cards? How to Check Real Capacity with H2testw!