volume could not be unmounted mac flash drive

Booted in Safe Mode, tried Disk Utility’s First Aid.After a forum search here, I tried (with varying success): Then I tried Disk Utility’s First Aid on both my 4 TB & 8 TB external USB drives, but neither would unmount, so First Aid failed on both. So, I ‘Quit’ iTunes when songs stopped playing. (I am not and have never been an Apple Music subscriber, not that there’s anything wrong with it.) The media files were all manually imported by me, they are not streaming songs from Apple servers. I could double-click or select files or hit Enter on selected songs/videos and their info at the top center of the Apple Music App window (e.g., artist, song title, artwork…) would appear, but no audio, no video and no time was displayed in this same window. After a few days adding more imported files without incident, today, after playing a dozen songs and music videos in the Catalina Apple Music App, it stopped playing. (I only lost 3,400 artwork files in the transition, but I'll be fine after therapy).

I successfully transitioned my old Mojave iTunes library into a working Catalina, Apple Music App library, and moved it to the newer Seagate 8 TB external USB drive. The internal iMac drive has an empty iTunes library.] I wiped/erased my internal Mojave drive (Disk Util Erase), installed a fresh Catalina (from a USB thumb drive) in a new user account (not my old Mojave account), then did a Migration Assistant from a Carbon Copy Cloner backup of the Mojave data from an external USB drive. I recently did a fresh installed of Catalina (10.15.7) on my 2017 iMac.

Volume could not be unmounted mac flash drive