Hi again Mark
I can't correct you, sorry. Like I said, I don't have any experience with this software, and in truth, not with any of these 'duplicate file finders'.
You have just upped the anti, as it were. If you were looking for a solution to just one file type, MP3's, then it's worth a try, but I now seeing you are looking at 10-15 years worth of backups for all sorts of files. The software I suggested is specifically for MP3s
You raise a very interesting situation that I have thought about for some time, and I suspect this will escalate dramatically in years to come with computer users like us. "What to do with all these old backups?"
In fact, your own problem fits exactly. We are all, now, aware of the need to Backup, Backup, Backup. We say that in these forums regularly, 3 backups of the same file, because when the original dies, (hard disk failure, malware corruption, whatever), one of those backups becomes the original and then we only have two backups. We make another one!
But 10 years down the road, and what do we do now? I have been making backups since 1995, starting on floppy disk no less. 3 different computers later, and I have no floppy disk drive. True, I can soon get an external USB floppy disk drive, but do I really want to? Are all those files I saved on floppy disk really needed now?
What about all those hundreds of CDs I have? And in 10 years time, what about all the external backup hard drives I have, if they are still working? Will I even care?
But back to your problem. You have 100's of thousands of files backed up. I wouldn't mind betting that many of these are 'system' files, for old computers you no longer have or use. If so, there is no way any duplicate file finder is going to recognise those and be able to tell you that they are duplicates of others elsewhere. Microsoft is well known for using the same file over and over again in its OSes, but updated for newer Operating Systems.
I'm not sure how we can help you with this task. I am going to have to consider my own backups, and perhaps even, think of radical, 'I don't need this 12 year old CD any more' type solution.
Good luck.
Mark