Hi,
On my 2 Tb external disk I had previously cleaned all files other than .flac and .jpg without success.
Done many other things found in this forum : no success.
I finally found a solution (for me) : there were corrupted flac files.
*** Running MP3tag, sorting by albums or by files : I discovered files without album or title name… Deleted the ones whose tags could not be edited.
*** More, I ran “Audiotester” ( vuplayer.com): it identified corrupted files with problems and showed the paths : I deleted them (yes, 100 titles, one by one…).
Guess what ? - Ext HDD library in Volumio completed in a few hours.
Hope this helps…
Enjoy !