Are you sharing the directory via nfs to your linux box ?
I would do some experiment to isolate the problem. But first I will try to schematize the scenario
and you in BoxB try to copy something to Directory_served.
Try in boxB to dd if=/dev/zero of=/directory_on_A/file
and see if it stops well stop it after a couple of gig
, if it stops before you may try to mount it manually and repeat the test.
If it doesn't fail again it's almost surely automount causing the issue.