Comment 9 for bug 1236619

Not sure, but this may be of help (at least for finding the cause):
I use Debian and experienced slow transfer and eventually hangs while copying large amonts both on CIFS and iSCSC (on the same server), with cp, rsync and Nautilus copy. But... with gnome-commander it runs smoothly.
In my situation it seems to be the speed of the server. With small amounts it's OK and 60 - 70 MB/s. With large amounts it started at the same speed, but then dropped, eventually to almost zero. Gnome-commander did about 30 MB/s, but carried on.
And now... mounting the server with the "sync" option gave the stability to cp, rsync. 30MB/s, but stable.
At my work, I compared a fast and a slow server. It's no surprise that the slow server did not collapse.
So my conclusions:
Slow servers probably get overloaded with the (default) nosync option. Th buffers just cannot handle the large amounts.
Adapt your options to the server speed and your needs.

Greetz,
Remco Siderius
Amsterdam, NL