All done now, many thanks to all on here.

Just to give a procedure, for anyone who might come across this in future, these are the specific steps I used (Windows 10):

1. Unwatch the internal ROCK media drive in Roon. Settings -> Storage -> Roon Optimized Core Kit Internal Storage -> three dots -> Disable
2. Access the ROCK web-based GUI and stop Roon Server. The current IP address on your network can be found by opening Roon on and looking under Settings -> General where it will appear under Roon Core. Enter the network address into a browser on a PC/Mac on the same network and this should open in a web page.
3. If you don't already have a drive containing a backup of the music files on internal storage, make one now - either to a NAS, or an external HDD, etc. (f setting up a new drive, the preferred format is exFAT).
3a. Access the ROCK internal storage on a PC on the same network by entering \\ROCK\ in the Windows Explorer address bar, then navigate to Internal Storage.
3b. Open the folder on the backup drive, or NAS, where you wish to backup the music files.
3c. Move the files accross (N.B. if you're a Mac user like me, be aware that dragging the folders across in Windows will MOVE the files, not COPY them as on MacOS, maybe right-click -> copy/paste instead, etc.)
(N.B. consider try connecting the PC and ROCK NUC to the same router direct, I had terrible speeds until I did this, networks vary, YMMV, so ignore or implement this part as needed.)
4. Once the backup is complete, turn off, open the NUC and swap out the existing drive for the new one.
5. Reconnect the NUC to the network and boot up.
6. Back on the ROCK web-based GUI, format the newly installed internal media drive.
7. Plug your external drive into a USB socket on the front of the NUC, if you used this backup method.
8. In Windows Explorer, access the ROCK internal storage and backup location (external USB, or NAS) as above \\ROCK\, etc.
9. Copy the files back over from the backup location to the internal storage.
10. On the ROCK web-based GUI, re-enable Roon Server.
11. Start up Roon, then go to Settings -> Storage and re-watch the ROCK Internal Storage drive.
12. Once Roon has completed importing, etc., you should be good to go!

HC: 861v4/ID40/LPS, DSP5000, DSP5000VC, DSP3100, SW1600, HD621, Oppo BDP-105, Sony VPL-HW55ES
2-Channel: Oppo BDP-105, TEAC VRDS-7/Trichord Clock 3/PSU, 551, Quad 303, Harbeth Compact 7ES-3, Kimber cables, Russ Andrews mains and supports.
Previous Meridian: 5000C, 5200SL, 7200.2, 6000, 562V.2, 568.2mm, 800v4, 808.2i, 818v2/v3, 861v8, 502