It's all done now. :-)
I formatted the external USB drive as ext3 last night, it worked without a hitch.
I have mounted the USB drive on the system in the garage and have exported it using NFS and mounted it on my desktop Fedora system.
I changed my overnight backup script in /etc/cron.daily to write across the network to the USB drive on the MSI system and that ran flawlessly apart from one base directory I had forgotten to create.
That works *much* better than trying to backup direct to the Freecom NAS drive (that is now the USB drive on the MSI system). Since the Freecom NAS only offered FAT32 lots of things failed to backup to it properly and I had to jump through hoops with rsync to get it to work at all. rdiff-backup just didn't want to know.