digitaldoc said: I would go with the WD 3 TB drive as it is only a little more. Yes, you can use 1 drive to backup 2 hard drives assuming you have enough capacity.
Paperdoc said: There is another consideration. Your plan is that, if the SSD fails, you will change to booting from the HDD. You can only BOOT from a HDD over 2 TB if your mobo has UEFI support. Does it?