Ok what you can do is this...
Disconnect the SD90 from the PC and reboot the SD90.
From the front panel - press the talk+xfade buttons. Press talk first and while holding the talk button and then press the xfade button ( top left).

You will be prompted to erase and to format the UserFs filesystem... select yes.

The UserFS is not a physical device, but rather what is called a loop device. It is a single file on the internal SSD that emulates a physical device. The above procedure will erase this file, recreate it and format it with the default folders/directories needed.

Let us know if this works for you..... more can be found here at the link below.

https://www.facebook.com/groups/KetronSD90