Thank you both. Got it to work!
Some other details. I set both PCs to use the same user name, which is a MS account. An both require password to log in and to let the network share.
An rather than use the drive map, I use the full share name of the other PC.
Share name is: \\Dell\Data$
Rather than block out the file giving me trouble, I used the wait and retry options thus I see what is going on.
I want the network drive to be copied to y local E: drive and I did this:
ROBOCOPY \\Dell\Data$ E: /w:3 /r:1 /s
it waits 3 seconds on a problem and ttys one more time. It snags some system files, but keep on going. and gets all the stuff I want.
The mystery is why would it try a system or hidden file or folder? Some types of system things in Windows should not be transported over a network. You would think Microsoft would have a default setting that would not transport information not relevant to drive content. Maybe there is a reason.
Anyway, now I can use ROBOCOPY to make a backup over the network rather than taking a portable external drive from one PC to another.
Thanks everybody.