I have a Windows PC now and an external SSD with about 25 albums in FLAC. Will the SSD work with a new Apple mini I wish to purchase as I read that Apple does not support FLAC. I plan to use the Apple mini with the SSD containing FLAC. I only use Roon and the Apple mini will be the core.
Whether the SSD works when attached to the Mac Mini depends on the file system format on the SSD. Most likely it is either exFAT or NTFS.
macOS can read NTFS but can’t write to it (with onboard tools).
exFAT should just work
As you probably want to add music to the disk in the future, you probably don’t want to use NTFS. Note that reformatting the disk deletes all files on it, so backup your music files before doing this.
If you reformat, use exFAT if you need compatibility with non-Apple computers. If you only attach the SSD to Mac’s, you can also use Apple‘s own APFS file system.
As for the FLAC file format, this is not a problem. The Apple Music app has the silly limitation of not working with FLAC files, but this doesn’t affect third-party apps like Roon.