Access Windows file share between workstations in a Domain and a Workgroup
Solution 1:
How are you connecting between the 2 machines, is it a mapped network drive, give this a try.
dos prompt on source, net use v: \destination\machine\share /user:username passwordhere
on the destination see if you can run a netstat (tcpdump), in addition on your watchguard device are you allowing file and print sharing.