I installed bootable windows 10 on my usb and lost all my previous files on the usb. How do I recover them?
Solution 1:
The installer has not only overwritten the allocation table, but also the actual space on the drive those files once occupied. You might be able to recover some of the original data, at least from the free space, using Recuva or similar tool.
A professional data recovery company probably would still be able to retrieve the overwritten data, but that's a very expensive process.
Updated as Silbee pointed out the shortcoming in the original answer