I changed drive letters. I saved all playlists to m3u format. I replaced all the drive letters in the m3u's to the new drive letter. I opened foobar.
If I try loading all playlists at once, foobar freezes.
If I load one playlist, it works fine, the song plays fine, reading from the new drive letter.
If I add a second playlist, foobar freezes.
If I place all the m3u's in the "playlist" folder, foobar freezes.
There is no change when I repeat the process with m3u8.

What is going on here?

it's not a solution to your problem, it's only a hint:
all my musicfiles are in a folder called "music", subfolders "artistname", subsubfolders "albumname"
all my playlists are in a subfolder of "music" called "playlists".
so far it's banal.
the entries in the playlists are:
..\artistname\albumname\01-songname.mp3 and so on.
no need to define a path, driveletters can be changed or the whole musicfolder can be moved or copied without ever renaming the entries in the playlists...