I am adding some information here relating to the following post, that I could no longer reply to:
'Delayed Write Failed' errors with USB Hard Drives
https://community.norton.com/en/forums/delayed-write-failed-errors-usb-hard-drives
I have found that the errors still occur even when most Norton features are turned-off, although the errors occur a lot less often.
So the errors could be caused by a PC performance issue - decreasing the load by turning Norton features off helps.
I think it is still woth having my 2 posts about this (including this one) online, because turning Norton off helped a lot.
The other thing that really helped was to start Windows XP in a 'Clean Boot' state before doing the copies to the USB drives, see here:
How to configure Windows XP to start in a "clean boot" state (Article ID: 310353)
https://support.microsoft.com/en-us/kb/310353
I hope this helps, because this problem was very frustrating for me.