"Now my assumption in my layman's terms is that the entry in the FAT still shows the correct size entries for each .jpg" ... what does Explorer show - are they credible file sizes? I ask because:
"It is also clear the available space on this drive increased quite a lot after the crash and chkdsk recovery"
You mention the FAT, but is the disk FAT or NTFS format?
I don't know what sort of software, apart from a selection of malwares, or crash, would rewrite thousands of jpg files and cause other disk errors. Corrupt a few, yes, depending upon your disk caching and what files were open at the time. And thumbnails, once created, are separate entities from the file itself.. if the viewing pgm can not present some thumbnails then it is likely that the file itself is null.
Look, try this software... it bypasses the MFT and tries to find file types you specify via their headers, but won't find fragmented files.
http://www.cgsecurity.org/wiki/TestDisk_Download
Unpack it, start PhotoRec from the exe itself in the win folder - READ the instructions in each winow!!
-Select the disk, then Intel,
- in the next window go to File Opt, pressing s will toggle all or none, so uncheck all boxes and then select jpg by pressing x, then b to save. Acknowledge the OK, then Quit.
-select the partition to search in, select Search,
-Other,
-Whole,
-navigate to and select the directory to save files into [using the arrow key to highlight the .. directory will enable you to work your way up the tree; when you can highlight where you wish to save TO, press Y]; the Search and copy will commence.
Simple, huh? And do not worry, this is a non-destructive tool. And you can Quit your way out.
!!!!Make sure you have enough space to save your files to.. the tool copies out immediately what it finds - another reason for selecting ONLY jpg files in File Opt.
!!!! Ignore TestDisk - it is not the tool you want, and can be destructive if misused.
Say how you get on...
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