First backup your music files to some external storage. Then install the new SSD and check it appears as a network share named:
\\ROCK\data\Storage\InternalStorage
I think the key step is to stop the Roon server "watching" while you make any changes to your storage.
Use the web UI (
http://rock.local/) and click "Stop" under the down-arrow next to "Roon Server Software".
ROCK continues to run so you can still see \\ROCK\data\Storage\InternalStorage and use the web UI.
Plug your external drive into the USB socket on the front of your NUC and copy files. I used Windows File Explorer. I had one
explorer window open on the external storgae, and another open on the NUC internal storage. Then you can copy/move/rename/delete
between them.
When done, click "Restart" in the web UI to restart the Roon server. Wait a bit to let it do whatever it does.
For example you might see some Library maintentance is needed under Settings > Library.
It is a bit stressful but I found it actually works well. Please check Roon support in case I missed something.