"I did have an interesting conversation with a technician in Maplins some months ago about the pros and cons of 500MB v.v. 1TB+ external drives. And he said that he would go for duplicate 500GB drives, since the new TB drives were just too fragile for archival use."

I wouldn't trust most Maplins staff with the technical knowledge to tell me what day of the week it was.

There are good reasons for using two drives instead of one, but in the context of this thread that's simply so you can duplicate your data and survive if one drive fails.