I don't know a lot about GarageBand - but I move most of ~/Library directories (where most of the User stuff is being kept) by simply linking them to a different place with:
ln -s /your/source/directory /your/target/directory in terminal.
for example, assuming that GarageBand is saving music to
~/Library/GarageBand/Music please move the directory first with
mv ~/Library/Music/GarageBand ~/Library/Music/GarageBand.bak/
and then try to link a directory in the place of
~ resolves to your /Users/username or
$HOME and is simply a shortcut to your home directory)
ln -s /Volumes/YourDevice/YourDirectory/GarageBandMusic ~/Library/GarageBand/Music
assuming it is the place where your data is being stored. You could try to create a hard-link without the
-s flag, but this is not possible for cross-device targets (i.e another hard-drive)
I'm sure you can do this, there are more ways to force macOS to use linked directories.
I don't know about GarageBand - but I keep my Mails, iTunes, Photos, Dropbox etc. on a different drive simply by either using symbolic links or, if necessary by hard-linking them to another directory.
For hard links workaround in macOS use https://github.com/selkhateeb/hardlink it does work very well.
Keep in mind that if you
rm -rf a symbolic link directory you delete link only.
When you do the same with hard-link on the filesystem then
rm -rf follows your link and deletes the things linked to it!
Let me know if this helps you.