Completed, but not really

[aepaep]

I have been trying to synchronize a large amount of data (a bit over a terabyte) from a network attached storage to an external USB drive. OS is Windows 7 64-bit, Core 2 Duo, 4 GB RAM. After some errors (not the fault of FFS) it finished and said "Complete" but there were still several hundred GB of files left (according to the FFS itself). If I rescanned (did not restart FFS) it would find all the files it hadn't synced (obviously I didn't check each file, but the size was the same) and would start the sync as expected when I started it. Again it got partway through (perhaps half) and stopped with the "Completed" dialog, which said how many files were synced and the number remaining (still many GB). Again I rescanned and synced, and it is in the process of syncing the remaining files. We will see if it finishes prematurely again (there are still a few hours left, it is a very slow NAS). Why would it be doing this? It indicates no errors, just gives the number of files synced and the number still to go.