Alan, it seems that either you or I are misreading what Bigbaby posted. He (let's assume) said that he selected one file and Recuva still required 3.5 tb of space.
In a normal scan Recuva will read the deleted entries in the MFT. Some may contain details of large files that no longer exist, so in theory there is a possibility that the returned total of space used might be larger than the actual disk capacity. It would be very, very unusual if on a 580 mb disk there were entries in the MFT totalling 3.6 tb. So unusual I would discount it. Of course the way to test this is to run a normal scan, which takes just a few mins, or secs.
In a deep scan Recuva will read sectors looking for a recognisable file signature. No matter how the disk is fragmented, or defragmented, or whatever the sectors are only read once, unless some other unknown factor is at play. So a deep scan would not return more data than the disk can hold. A deep scan also runs a normal scan first.
If the deep scan is returning a total of 3.6 tb to do a recovery from a 580 mb disk (I don't know how you can count the amount of data without attempting to run a recovery) than I think that the other factor, whatever that is, is present.
I haven't the time to do too much thinking at the moment, but it looks as if the disk is being read several times. I wonder if the disk somehow is being allocated to several disk id's, f, g, etc, even if they're not showing? This is only initial thoughts.
I would suggest scrapping the deep scan. Reboot the pc and check the disk allocation (you have reported problems with this). Do this until you get a clean boot and disk alloc.
Run a normal scan, see how long it takes and how much space it needs for a recovery (you don't have to run the recovery).
Then run the deep scan again. It should take no longer than an hour or so. If you notice that stage 1 has completed then cancel stage 2 (or 3) of the scan, You will still get the results and it will save time. Then report back.
Oh yes, run Recuva in Advanced mode, it's so much easier to manage. And check that in Recuva the disk you're scanning isn't set to All Local Disks.
PS I'm assuming the disk is NTFS, and your O/S is?