replacing failed hard drive

Hi. Yes it is a backup of another readynas. In most cases I actually have several backups of my files, however there are some files on the backup readynas that are only on the other readynas. For example I bought a lot of courses from greatcourses.com and I only have them on the first readynas with a backup on the second readynas, mainly because they take too much room on my computers...same thing with about 1 tb of music I bought and ripped from cds, but I recently made copies of my music onto external drives. I prefer to have more than one backup of my files at any one time, which is why I have started to make second backups on external drives but I just started doing that and haven't finished yet. However if the second readynas fails to boot up for some reason I think it's very unlikely that the other readynas will fail before I can move the rest of my files onto external drives, so I will insert the new disk, then if not recognized I will reboot the readynas. Thanks again for the help.