It's unfortunate, but I think those of us who don't understand technology all that well suffer from something I like to call "The CSI effect". In a lot of popular media, fictional stories tell us that data, once it exists, is always recoverable if you just know the right trick to get it back - and that is not always true.
When you delete a file on just about any computer system, the file is not truly deleted in the sense of its physical presence on the storage medium. Your hard disk (or SSD) simply labels the file as deleted, but it's still present on the file system. This is where recovery or "undelete" software comes into play. That kind of software simply scans for orphaned files that match the search criteria. Once it's found, the flag is simply reset and the file is once again made available.
What can happen in the intervening time between when the file was "deleted" and the recovery software is run, is that once a file is marked as deleted, the space it occupied is now able to be overwritten. So, if you delete a file that you need back in a pinch, your best bet is to try to recover it ASAP, particularly on a crowded disk.
So, as you can see, recovering deleted files is already a fairly delicate, and sometimes unpredictable process. When you add in a multipass erase, you further complicate things. Yes, you made a backup of the disk at one point, where the file may or may not have been in a recoverable position. However, most backup software does NOT backup files that are orphaned. In order to get a file that was flagged as deleted, the backup software would need to do a perfect, bit-for-bit backup - that's just not how most backup software works. Rather, it backs up currently present files that are detected as needing to be backed up. In many cases, if a backup was previously done, it will in fact only get files that were added or changed since the last full backup, in order to speed the process.
This is why you're getting the kind of incredulous response you may not have expected here in this thread. While it's true that various government organizations or sophisticated hackers may be able to piece together bits of information on a scrubbed disk, it's pretty hit or miss, and not fool-proof. And even if there is data that is recoverable, it will likely not be in terms of full, coherent files.
I'm not sure of your reasoning for doing a multi-pass erase, but I would suggest that if you have certain files that maybe you'll need down the road, but for the moment don't want to be seen, that you instead look into options for encrypting the files, rather than deleting them.
awww man!
