Har brugt denne metode flere gange med succes;
(indholdet slettes!!)
How to Restore a USB Flash Drive using Diskpart in Windows.
Open a command Prompt as administrator (cmd.exe).
Type Diskpart then press enter.
Next type List Disk then press enter.
Type Select Disk X (where X is the disk number of your USB drive) then press enter.
Then type Clean and press enter.
Next type Create Partition Primary then press enter.
Type Format fs=exFAT Quick then press enter.
Note: (for 32GB and smaller drives, use fs=Fat32) instead.
Then type Active and press enter.
Finally, type Exit then press enter to Quit.
https://www.pendrivelinux.com/restoring-your-usb-key-partition/