How do I "unpurge" the undo function?

I'll just add that one of the reasons Dave is recommending incremental file names, is that we see regular posts to this forum, from people who have lost work when something goes wrong while saving a large PSD or TIFF file.  If your laptop battery fails, or Photoshop or your operating system crashes.  If the PSD file is part way through overwriting the last version you saved, then the whole file is toast and gone forever.   About the only good that comes out of such situations, is that we get to learn from other peoples bad luck, and adopt the incremental file name procedure.

We discussed an alternative which is to use Dropbox which keeps a history of your uploaded files for 30 days.  If you have a fast internet connection, then this very doable.

Incidentally, at that time I said that I'd heard that you could expand your Dropbox space to 10Gb by installing Dropbox on multiple devices and computers, and logging on with your ID. Every time you do this you get an additional 1Gb up to the maximum 10Gb of free space. I'd been told that having done this, you could then forget about the other devices, and that you still had all 10Gb.  However, someone else has had the experience of the additional space being taken away without warning, so you can't rely on it.