[SOLVED] virtualbox backup fails

Hi All
I am fairly new to ubuntu and Oracle virtual box and have the following problem.

Details first - using Ubuntu 10.04 and Oracle Virtual Box 4.1.12.r77245

I was asked to set up a backup of the virtual boxes on this machine every week.

I've set up a script, which checks which vms are running, powers them down in turn, rsyncs the files to a separate storage area and then restarts the box.
This is cronned and logs output to a log file.

When run as a simple command in the shell - no problem.
However when running via cron the boxes restart but are inaccessible, so I then have to power them down and restart them from the host.

So I thought instead of using the poweroff option, let's try it with the acpipowerbutton option.
Tested this and the machines came back up but with a locked session on each.

Has anybody any ideas as it's severely bugging me - i've looked at tkms etc, but no joy.

Code can be pasted along with error messages (none in the logs).
All help appreciated - as I'm sure people must be backing up their vms all over the place

Thanks