First, get a "new" reliable USB flash memory stick. Perform a complete backup of your MODX to ensure you do not lose anything from this step forward. This will save an X8A backup file on the USB stick. Make a copy of this X8A backup file on your computer's hard disk. After you have safely stored a complete backup of your MODX - anything you do from this step forward can be undone/restored/fixed if necessary.

Later found the library located under the piano area.

I don't understand what this means. What "piano area"?

Was this library distributed by someone else (Yamaha, or other 3rd party provided for download for promotion, purchase, etc) -or- was this a Library you created?

The flashdrive no longer reads the flashdrive.

Does this mean the USB TO DEVICE port on MODX no longer recognizes the USB Flash Drive you previously used to install the Library? Does the USB TO DEVICE port read other USB flash drives (different ones?).

What's your computer you use to manage (copy, format, etc) your USB flash drives? Windows, Mac, other?

Is your MODX out of waveform memory? Are all 8 Library slots occupied? If you press the [UTILITY] button then navigate to contents/data utility - you should be able to see waveform memory usage:

... and you can look at the Library usage ...

The next screen after touching "Library" will show all Libraries loaded, memory consumed by each, and - in the upper-right - how many Library slots are used (#/8 ) where # is the number of Libraries loaded.

If the problem is only the ability to read a USB stick - then that is remedied by finding a different USB stick that can be read. The answers above slightly inform some of the specific suggestions for a route to take.