Manual Back-Up of Music Library to External HD missing some Folders/Files, why??

(I am fully up to date on iOS and refer to Apple Music as 'iTunes' because I'm old...Deal with it!)

I have a huge library of music, it grows almost every day and I use iTunes to keep things labelled, organised and correctly tagged. Some time ago I maxed out my HD so moved the majority of my 'Archive' to an external HD which I manually add to each month or so.

I begin by opening up the iTunes library to show all the Artist Folders. I then open up the Folder on the HD containing the Archive. I then systematically work through the Alphabet, A, B, C etc, highlighting the artists beginning with that letter and right-click-copying them, then pasting them to the External HD. This then brings the option to 'Skip' folders that are already there, 'Replace' or 'Merge' when new files are present in the iTunes library but not on the HD... So far so good.

I have noticed thought, that sometimes not all the new files/albums/folders are recognised and copied - some not even picked up on the Skip/Replace/Merge step, why is this?? I feel like I've been left with an incomplete back up of my precious collection, which when I remove older material from my MacBook HD to make room - thinking it's backed up - I find it's not and it's gone forever?

Why is this happening and why can't anyone at Apple tell me how to stop it from happening?

FYI: There are no error messages, no corrupt files, nothing running alongside iTunes & ExHD to cause a problem etc... Just a simple Copy & Paste... but untrustworthy.

I do also use Time Machine to back up the MacBook, but obviously once music is removed from iTunes it won't appear in a Time Machine backup so if it doesn't copy manually then I lose it...

Can anyone help explain what's happening and help me solve this issue? I'm getting more and more disheartened with my beloved Mac...

Thanks,

Dave