Music can't find my music files on external drive

I had about 2Tb of music files [classical music, mostly uploads, some purchased and some personal recordings] all curated in playlists [by Composer / Work / Performer etc] and stored on an external HD.

Today this failed [see below *]. I've been a chorister for 60 years – music is in my soul, so this matters to me.

Fortunately I have a backup [complete and updated just 12hrs ago]. So I went in to Music Settings and changed the Music Media folder location to point to the folder containing the backup files. A quick Get Info tells me there are 58k items using 2Tb on disk

.

However, when I look at any playlist, the tracks all show the dreaded ! symbol – can't be found.

Manual re-naming of every track is obviously not an option, so how do I re-connect them en masse?

*The HD is an elderly but hitherto highly reliable LaCie P9230 USB drive [5Tb]. Today it didn't mount, so I disconnected / reconnected first the power lead then the USB. It whirred briefly, made a few clicks and then fell silent. Doesn't show in Disk Utility.

Help!