My Nucleus+ suddenly doesn't see my external SSD storage drive

[Suedkiez (Just a fellow user like you)]

What’s the disk format of the disk, Windows’s NTFS?

Did you have it attached to the Windows machine and maybe unplugged it without doing a proper “remove safely”? Did you recently install the RoonOS update for the Nucleus?

If yes:

ROCK USB Connection Issues? [Investigating - Ticket in]

NTFS? We upgraded the NTFS driver to something far more stable, and less prone to corruption. Unfortunately, this driver is far pickier about drives that have been improperly ejected. The solution, if your drive is NTFS, is to plug it back into a Windows machine, run chkdsk /f on it, and then eject it properly and put it onto the ROCK machine. We are looking into a safe solution that will avoid this chkdsk /f.

Safely remove hardware in Windows - Microsoft Support

And welcome to the forum