Most Valuable Professional
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If shadow copies were being made of that external drive, a tool like ShadowExplorer would allow you to view them and potentially recover files out of a snapshot taken before that deletion occurred. Short of that, you'd be looking at file recovery utilities like Recuva, but there's no guarantee that the recovered files will be intact, especially if your retention policy is set to run before the new backup rather than after. There's nothing built into Reflect or Windows itself that would help with this, though. Going forward, you might want to consider changing your retention policy to retain at least 2 Full backups. You can always manually delete a Full if you don't want to keep that many, but that retention policy setup would prevent Reflect from ever doing this automatically, so you wouldn't have to worry about specifying a different destination folder.
On a side note, unless you're replicating your backups to another location, only capturing a Full backup once per year is a very high-risk strategy. If something happens to that Full, then you lose an entire year's worth of backups, which could potentially mean all of your backups if you only ever have one Full at a time. I would consider capturing more frequent Fulls if at all possible, even if it means buying more storage capacity. If that's simply not an option, I would at least the very least perform frequent manual verifications of your Full backup to make sure that you can restore from it if ever required. Even if you verify your backups when they're created, that doesn't rule out a file system or hardware-level issue creating a problem accessing that file later. If you ever saw a failed verification, you would at least have a chance to create a new Full right away, so your only risk would be something going wrong with that file and having a major data incident between verifications, in which case you'd still be stuck.