I really love this program but I always have a problem with saving changes of larger files.
I'm accessing my files through junction points on a very small drive (40MB total / 10MB free) (this overrides the drive letter and path length limitations).
The trouble now is, while mp3tag is checking the free space of the drive of the path it's impossible to save files larger than those 10MB though the actual location has more than enough free space. An old workaround was to access these files directly on their 'residential' drive but sometimes the paths become too long and meanwhile I ran out of letters.
I think a better way might be to check the free space of the directory itself where the file is located (not the start directory as possible sub directories might have different locations!). This would also eliminate complications with disk quotas.