I think there was above 200MB showing in Properties and I checked it very carefully each time I copied a folder (with many sub-folders) to it. It tested okay each time except for the last one.
There's something nice about data all being in one place. Some-thing singular. I agree there are lots of reasons to use more discs.
My next go seemed to be okay - I pulled the disc several times letting that message come and go. I left about 1.5 Gig spare.
In haste, cos I'm running Recuver on the duff HD...just for fun.