f3probe

Probe a block device

$ sudo f3probe [path/to/block_device]

$ sudo f3probe --min-memory [path/to/block_device]

$ sudo f3probe --time-ops [path/to/block_device]

f3probe [-h|--help] [-V|--version] [-s|--time-ops] [-d|--destructive] device

-h, --help
    Display help message and exit

-V, --version
    Display version information and exit

-s, --time-ops
    Show time spent in each operation (write/read)

-d, --destructive
    Required for real probe; enables destructive writes to detect true capacity

device
    Path to raw block device (e.g., /dev/sdb; not mounted)

f3probe is a Linux utility from the F3 suite designed to detect the real storage capacity of flash memory devices like SD cards, USB drives, and eMMC, which are often counterfeit with inflated reported sizes.

It performs a destructive probe by writing unique sequential patterns from the end of the reported capacity backwards toward the beginning. After each write, it reads back the data to verify integrity. When readback fails (due to non-existent flash cells), it pinpoints the actual usable capacity. The tool reports the tested size, real size, and percentage of fake storage.

Key features:
- Extremely reliable for identifying fakes.
- Works on raw block devices only.
- Probe time scales with device size (hours for multi-GB drives).

Non-destructive mode is available but ineffective for capacity probing—it only checks the reported size without writes. Always use --destructive for true detection.

Ideal for verifying new or suspicious flash media before trusting it for data storage.

Destructive mode erases all data—backup first! Do not use on HDDs, mounted filesystems, or partitions. Can take hours/days for large drives. Unplug other USB devices to avoid errors.

Reports: Tested: X MiB
Real: Y MiB (Z% fake)
Example: Real capacity: 100% tested, no fakes.

Available in most distros: apt install f3 or dnf install f3. Source: https://github.com/AltraMayor/f3

Developed by Michel Machado starting ~2012 as part of F3 suite to combat fake flash drives flooding markets. Actively maintained; latest versions improve speed and compatibility with modern devices.

f3write(1), f3read(1), f3fix(1), badblocks(8)