I've forgotten something...
Before to fix your large mp3 files library you have to find what mp3 are corrupted: download
http://www.jumpingcholla.com/jce_trapper.htm MP-3Library v2.20 (if you know a better mp3fixer please give me a shout) & install it freely. Go to "Settings" pad\"ID3-Editor/MP3 Analysis" pad and activate
all the options in "MP3 Analysis" settings and go back to Files pad. Go to "Import Audio Files (including subfolders)" command, than to Menu Edit\"Analyze all MP3s (in FILES)", here you'll be probably surprised to find so much problems in your files...
In first fix the VERY corrupted files (with red flash), usually less, than if you really have many files with minor problems I suggest to come back to MP3Gain, and select the same Folder and subfolders than in MP3-Library.
Select all files in Mp3Gain, right-click on selected files and activate "Remove tags from files", when it stop to remove its tags close it and come back again to MP3-Library to make a new analysis of all files.
Surely and lukily will remain only a bunch of files found with probs in the Result pad and to fix one to one with MP3-Library.
I really hope that this suggest could save to you all the time I spend to fix my files before to understand that was a MP3Gain issue...
Regards