I get the errors that either the file is inaccessible or different

Nero requires a plugin to be able to work with mp3 files. This plugin has to be purchased seperately. Without this, you'll first have to convert your mp3 files to wav manually before writing them.

On another note : maybe you can use a CDRW disc for your cd wrinting experiments ? Then you won't end up with a stack of unusuable coasters if the problem does end up to be the drive itself (I doubt that though, I've never known a Plextor to go bad and I've sold dozens of them. But I suppose it can happen)