How do I tell one My passport from another?

I own two of the “My passport” devices, one purchaed this past year, the other a number of years ago. Obviously they are different capacities but the label doesn’t indicate which one is the largest and most recent one. I use one for backing up my desktop and the other for backing up my laptop. Unfortunately, after unplugging each from the computers, I don’t know which backup device goes with each computer.  How can I tell which one is the older one? Any suggestions? I"d also like to know how one would reformat one, if needed. 

Put a big sticky label on top of each and write on it which one is which?