Is the USB SSD formatted in a format that the Nucleus can read? I believe the preferred format for compatibility with RoonOs and other operating systems is exFAT but the RoonOs can also read ext4 and NTFS (although the latter can cause issues - see the next point).
If the USB disk is formatted with NTFS, what is correctly ejected from the Windows computer before connecting to the Nucleus? If it was not, it may be unreadable. This can be fixed be reconnecting it to a Windows computer, running a filesystem check (e.g. “chkdsk /f” from a Windows terminal command line), and then properly unmounting the USB drive before removing it from the Windows computer. Alternatively, after the running chkdsk, shut down the computer and then remove the USB disk.
Have you changed the watched folder settings in Roon? The new USB disk will have a different folder within the Nuclues’ ‘Storage’ folder compared to the old disk. The watched folder settings in Roon will have to be updated to match. In the Roon client, look at Roon → Settings → Storage and check that the correct folder has been added. If it is not present, add it, and, if no longer available, remove the folder that was associated with the old USB disk.
Welcome to the forum! Is the USB drive by any chance NTFS formatted? NTFS is not suitable for Linux-based operating systems, such as the Nucleus. To resolve the issue with NTFS, you can use @Wade_Oram 's suggestion, here it is in step-by-step format:
Connect it to a Windows PC
Open Command Prompt on a Windows PC
Type chkdsk d: /f (assuming windows has assigned the external drive D)
Safely eject the drive from Windows task bar (icon may be hidden in the bottom right)