Secure Wipe of Free Space on USB Drives and SD Cards

Because a browser cannot write into an existing file in place. The File System Access API writes every change to a new copy and swaps it in, so the old bytes stay on the drive. Filling the free space overwrites those bytes wherever they are, which is exactly what cipher /w on Windows and sdelete -z do.

As long as writing the whole free space takes. A 32 GB flash drive at 20 MB/s needs about 27 minutes; a USB 3 stick or an external SSD is several times faster. The run ends by itself when the drive reports it is full.

No. The filler files are removed once the drive has finished writing, and the free space comes back. Files you did not select are never touched.

Not from the phone: phone browsers do not offer the File System Access API, so the page can neither read nor write anything there. Put the memory card into a computer and open this page in desktop Chrome or Edge, or reset the phone from its settings — modern phones encrypt their storage, and a reset destroys the key.

Flash controllers spread writes across the chip and keep reserve blocks that software cannot address, so a copy of old data may survive there. SSDs with TRIM may have discarded deleted data already. For certified destruction use the maker's secure-erase command or full-disk encryption.

The free space of the whole volume that folder is on is filled, so the system drive will be full for the duration of the run and programs may complain. Use it on removable drives, or close other work first.