Used to use a dongle with Autocad and yes they were a pain (I think it may even still be in a drawer here somewhere)

I use a 250GB Western Digital passport which is a small 2.5" drive for one of my backups and this comes with me when I am away from the computer. This only needs one USB lead to work. If you use one from a hub make sure it is a powered one (and the power adapter is plugged in

)

I also use two other external drives which I backup to twice a week and the main computer has two HDDs and backs one to the other every 15 minutes. Both these have separate power supplies.

This is probably over the top but as I use the computer for work losing a directory full of CAD drawings is not a cheerful prospect.

If you are going for an external drive I would buy new, and either a Seagate or Western Digital as backup is a fairly critical operation. If you think that 40Gb will be large enough then I would look for a min of 80Gb, more if you intend to run more than one backup file-it will soon fill up