If the USB flash drive used to work but it no longer does, it could be that the filesystem has become corrupt. Plug the USB flash drive into your computer and move the files off from it. Format the flash drive on the computer as FAT32. Plug the flash drive back into the mixer and check in System> USB Memory, try formatting the flash drive again on the console.
If this fails to work, be sure that you are using a Roland approved USB flash drive. Not all flash drives are supported.