"File "C:\Users\###\audio\book 3 Part 1\01 Track 01.mp3" cannot be renamed to "C:\Users\ ###\audio\book 3 Part 1\Half Life - 01.mp3". The process cannot access the file because it is being used by another process. "
I have checked, no other program is running, no other program besides mp3tag is accessing this file.
I have tried with 1 and multiple files, always the same.
Have you tried to use a tool to check that the file in question is really not being used by another process? I use Tag-Filename a lot and I never have any issue with it. You can use Handle from Microsoft or Process Explorer (GUI version) to check the file in question.
It could also be a permission issue, perhaps try to start with "Run as Administrator" by right clicking Mp3tag.exe and retry the Tag-Filename operation.
I hate to sound like your friendly neighborhood tech guy (and it is a drastic measure), but sometimes you just have to do a restart of your computer for all hooks into certain files to be relinquished.