Hi @Sernyl_S7 ,
Thank you for the answer.
Is it this one the post you were referring to?
If yes, the problems you found were related to
A) incomplete tags (no album name or title)
B) Corrupted files (FLAC)
I’m not able to use Audiotester 
(I’m on Mac osx)…
“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 !”