I don’t know about the amp you have but if I were you I’d do a quick google search to find out how others have got on. Some systems can be more trouble than others over numbers of folders & files and how the disk is formatted.
If your music collection isn’t too massive I expect it will be easier to use a good sized flash drive.
If you need to use an external disk that should work just the same as a flash drive and if it doesn’t draw too much power the amp may manage fine. If not you can use a Y cable to add power.
Are you anywhere near Worthing or Farnborough or somewhere between? If so I have an old drive I used to use in my car you could borrow to see if it works.