The Auto Recover feature is not compatible with 64-bit versions of Windows 7.
As for what happened with your file: ZBrush is still a 32-bit application (until ZBrush 5 comes out). As such, Windows imposes a 4 GB maximum for the size of files that it can load. If you ever start pushing close to that, it’s time to rethink things a bit!
One thing that will help you a lot is to not use the ZPR format (via File>Save As) unless you specifically need to. For example, if you have render settings that you need to keep and don’t want to redo or if you have a need to have your work as multiple Tools rather than SubTools. The ZPR format saves just about EVERYTHING - including any incidental Tools that you might have created during your work, materials that have been loaded, render settings… you name it. As a result, your file sizes can easily bloat if you’re not being careful.
Instead, use the ZTL format (via Tool>Save As). This saves the current Tool and its SubTools, applied textures, etc. In almost all cases, this is all you need for saving your models. You’ll have smaller files which not only helps avoid exceeding the Windows limitations, it also helps ZBrush work more efficiently so you can avoid those out of memory situations.
By the way, it’s bad practice to save over your most recent save. To stay on the safe side, it’s a good idea to save two or three incremental versions, overwriting the oldest with each save. That way, if you do hit a situation like this or a file corruption you should be able to revert to the last version or the one before that. You’ll lose less work if the unthinkable happens.
