I don't have a backup software. I just use an external HD that copies my files onto the external HD. The HD just plugs in with a USB and anytime it is plugged it, it backs up all the files I have selected onto the external HD. This is done for protection...
But, in the case of a fire the computer and HD are both at my desk making the backup pretty much useless if they both burn up. So, I want to put the HD in a safe. But, if it is in a safe it can't do a backup. SO, I want to have two HDs so one is plugged in and one is in the safe and I will alternate them so one is always safe.
I want to know if I can use the same exact HD and if I alternate them the computer will see it as the same HD.
Which external HDD are you using? Are you talking about Time Machine backups?
From the sounds of things, you should be able to alternate the drives without any foreseeable hiccups, but without more information it's impossible to say for sure.