Another thing to be sure of is buying a proper branded one, not some no-brand el-cheapo unit that costs next to nothing.
You sometimes get units that say they are a certain size (32GB, 64GB, etc) but then fail to store your data properly ... transpires that they end up being much less than the reported capacity.
Have a look here:http://www.makeuseof...now-buying-one/
Also, this is worth looking at:http://www.instructa...sh-Drive-Scam!/
After backing your valuable files up to the drive, be sure to check that they copied correctly using a file comparison tool such as WinDiff.
It defeats the object of having a backup if the backup is corrupt or not taken correctly at all!