Big issue with backup/restore

Normally, 7-Zip doesn't generate the file properly if there is not enough space or it can't get all the files. It will error out and not produce the file. And the backup utility will present an error. I'd wager that 7-Zip can't detect the file size restriction and simply dies, leaving the file incomplete and, thus, corrupt. The backup utility then sees the file and thinks all is well, relying on 7-Zip to let it know there was an error.

I don't think anyone's tested the backup utility with the combination of (1) a large amount of files that will produce a >4gb 7z backup and (2) backing up to a drive that is formatted as FAT32. I know I haven't as all my drives are NTFS except for my smaller flash drives. And I know we didn't really consider this situation as we'd planned on people backing up their flash drives to their local PCs, which were assumed to be NTFS. I'll have to re-examine the code to determine if there is a way to properly address this (perhaps disallowing larger backups from being saved to FAT32 at all to avoid this possibility). This will be less of an issue as we've dropped support for Windows 95/98/Me; and Windows 2000 and up should be using NTFS. But it could still occur.

As for your case, I don't think there is anything we can do to retrieve your files. As the 7z archive is incomplete, it would seem that whatever is in it is what was restored. You may be able to undelete some of the files from the portable device using one of the undelete utilities out there, but some may be overwritten by the restore to the stick you already did. And if you didn't reformat, the current file system is probably unstable anyway (anytime you get large amounts of corruption on a drive, it's a huge red flag).

I wish there were something more I could suggest or do to get your data back, but I can't think of anything besides that. We call 7-Zip command line from the restore utility (as well as backup) and it looks like it is restoring what it can. 7-Zip GUI should be able to restore the files that are actually present in the archive, but will show an error for any that aren't. It is possible that some of the TB files are there but some are not, so you could try some various directories to see if they are there.

Does anyone else have any thoughts on how we can help Achim out?

Rule number one when you have a corrupted anything, is make copies of all the files, including the corrupted ones. That way, as you try this or that attempt to repair, you try it on the COPIES - if you mess stuff up, you still have the originals, and you can make another copy and try again.
So, make copies of the corrupted zip files, and start looking around for software that can help you recover.

A quick Google search got me this:
http://www.essentialdatatools.com/products/objectfixzip/

I've never used it, but it seems to be what you need.
There are likely to be more.

You may want to try several different, in case they each only recover a a part. That could maximize your recovery.

I'm very sorry to hear of your issue with your emails. As others have said, the situation may not be recoverable.

Suggestions for future solutions aren't much consolation, I know.

Something that everyone should take away from your experience is that backing up is only half of the data protection effort. Restoring is the other half. Backing up without periodically testing the restore places an awful lot of trust into the process.

Again, sorry to hear of your data loss.