Replication - I/O Error - What is wrong?

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Greetings Freenas community.

I am hoping that someone can provide some guiding light as I have run out of ideas.

I have two Freenas servers and trying to replicate one folder to the other. I have setup an SSH tunnel which works between the two servers. I have configured the replication task in the GUI, however as soon as the replication begins I get an error "Broken pipe cannot send 'gandalf/projects': I/O error". I have searched for an explanation of "I/O error" to no avail. Extract from log file is as follows :-

Code:

Jan 15 19:00:02 freenas autorepl.py: [tools.autorepl:264] Creating NAS01/Nick/projects on remote system
Jan 15 19:00:02 freenas autorepl.py: [common.pipesubr:49] Executing: /usr/bin/ssh -i /data/ssh/replication -o BatchMode=yes -o StrictHostKeyChecking=yes -q -p 22 ***x.yyyy.com /sbin/zfs create -o readonly=on -p NAS01/Nick/projects
Jan 15 19:00:02 freenas autorepl.py: [common.pipesubr:49] Executing: (/sbin/zfs send -R gandalf/projects@auto-20130114.0900-2w | /usr/bin/ssh -i /data/ssh/replication -o BatchMode=yes -o StrictHostKeyChecking=yes -q -p 22 ***x.yyyy.com "/sbin/zfs receive -F -d NAS01/Nick && echo Succeeded.") > /tmp/repl-84588 2>&1
Jan 15 19:00:03 freenas autorepl.py: [common.pipesubr:42] Popen()ing: /usr/bin/ssh -i /data/ssh/replication -o BatchMode=yes -o StrictHostKeyChecking=yes -q -p 22 ***x.yyyy.com "zfs list -Hr -o name -S creation -t snapshot -d 1 NAS01/Nick/projects | head -n 1 | cut -d@ -f2"
Jan 15 19:00:03 freenas autorepl.py: [tools.autorepl:323] Replication of gandalf/projects@auto-20130114.0900-2w failed with warning: cannot send 'gandalf/projects@auto-20130114.0900-2w': Broken pipe cannot send 'gandalf/projects': I/O error

Please can someone provide some assistance in determining what is wrong.

Many thanks
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