ROCK installs the Roon OS and Roon Server software onto the m.2 SSD in a NUC.
If your NUC is a “high” model, it has extra physical space inside it to take a 2.5" drive (SSD or HDD) that can be used for storage of music files.
A ROCK/NUC system can also support external USB drives.
What is the problem about attaching your new USB SSD to your ROCK/NUC system?
But, but, as I said, the Roon OS and Roon Server has always just been on the m.2 SSD, and USB drives are just used for storage - so nothing has changed…
Please read the article I linked to earlier. It contains the statement:
We recommend USB drives connected to ROCK be formatted exFAT if you require compatibility, or EXT4 if you don’t.
While Roon OS will work with NTFS formatted drives, these require careful handling - just unplugging them from a Windows PC without “Safely removing” them is likely to cause Roon not to be able to read them once reconnected to the NUC.
When you unplugged the old drive did you disable or remove the storage location in Settings/storage in Roon for the old external drive?
Did you add a storage location to settings/storage in Roon for the new external drive?
Once again, read the article I linked to. In particular the section on External Storage (USB). It describes the steps to add a Watched Folder on a USB drive.
I have experienced this issue several times when removing my USB SSD for other operations such as backing up purposes.
@Geoff_Coupe is completely correct here.
Even though I shut down my Roon Rock machine first this still occured.
Edit: also safely removed from Windows.
If formatted with NTFS, leave connected at all times.
I believe the partition table is altered and therefore Roon Rock can no longer detect the SSD. Happy to be corrected here, but that is all I could come up with.