Hi Roger,
if you want to use a USB storage option, I’d recommend initialising it first in the Synology DSM with a ext4 filesystem. (Control Panel -> External Devices: Select your USB Storage and select format on the top bar). This process will delete everything on that disk!
USB devices will automatically be listed in the shared folder list as usbshareX. To use it as a storage for Roon Server’s database, you need to rename it to “RoonServer”. This can be done by selecting the usbshareX entry in the list and click the “Edit”-button.
Afterwards you just need to install the spk in the Package Center (if not already done before).