I'm not sure if it goes to sleep in the interim, but I shall keep an eye on it from now on and we'll see.
In the meantime I'll have a look at the thread you suggest.
Many thanks for your help.
It backs up OK but after a while 'Disk not ejected properly' pops up, and the only way to get it working again is to restart the computer.
Amazon have exchanged it, but the new one does the same thing.
Thanks in advance.
MDS