Figured it out after I plugged it into my PC.
The Mac likes to add metadata files, so it had made dotfiles with metadata for every single audio file. For example:
for “asong.mp3” it created “.asong.mp3”
I take care of this issue at the command line: I have the Clip+ hooked up to my Mac. I Finder-copy mp3 files to a folder on the Clip+. Then, in Terminal, I switch over to the folder where I copied the mp3 files. Then I execute this command:
$ find . -type f -iname ‘.*’ -print0 | xargs -0 rm -v
This will delete all files in that folder whose names start with a dot, i.e, the ones like
._01 Song title.mp3
that confuse the Clip+.
If you’re not comfortable with the command line, you’ll need to get someone who is to help you with this the first time.
Once you get it down, it’s very simple.
WARNING: Never execute this command anywhere except in the folder where your mp3 files are. Ignore this warning at your peril.