Need help with new HDD

If you have not updated the OS in between, then you definately can do that.  If you do update the OS on the NAS, it is supposed to simply install the new one on the old drive when you put it in and move on, but there is some risk.

If I read your description correctly, you are only using one drive at a time.  If that's correct, you could have added the second drive as a separate volume along side the first.  It's too late now, unfortunately, because both will be named "data" and you can't have two with the same name.  Netgear provides no capability to rename a volume.