Photoshop 8: Error creating backup. Negative total media size. Error writing file(s).

I am trying to create a backup for my catalogue in Photoshop 8.  I need to create a full backup of a large catalogue.

When I start the backup Photoshop calculates the total media size.  Initially it reports a size of 0 MB.  As soon as I select a destination drive this total changes - to a negative value!  -8,796,093,022,113,50 MB, to be precise.

If I ignore this and proceed with the backup it all appears to proceed normally until the end, at which point a pop-up appears that says (only) "Error encountered while writing file(s)", with no further information.  The backup destination folder at this point contains a large number of small (1-10 KB) .xml and .xmp files, a large number of video and image files (.mov, .avi, .mpg, ,bmp, .jpg, .gif, .tif, .psd) and a few single files: a .dat file, a .pref file, and a catalog.buc.

I have checked the write permissions of the folders I believe Photoshop is using for the backup, and everything appears OK.  I have also tried running the backup as an admin user, and still encountered the same problem.

Any ideas?