firstname lastname wrote:
Disk Utility has been emasculated in El Cap -- Can't repair permissions anymore, on any drive. Onyx etc only allow on the boot drive.
There are no permissions on a FAT volume. There is nothing to repair. You have some other problem.
All drives are mounted to /Volumes/
If you are writing to a directory in /Volumes/, but it is not actually writing to the flash drive, you have a "phantom" mount point. Eject the flash drive and see if the mount point for it still exists in /Volumes (something like /Volumes/flashDriveName/).
If it does, copy all of the files in that directory to another folder on your Mac as you may need to copy them back onto the flash drive if we can make it work again.
Once you've copied the data out of the phantom mount point, delete that folder--Make sure you are not confusing it with a drive that is actually mounted.
Then, reattach the flash drive and see if it mounts correctly and you can read/write.