Someone correct me if I'm wrong, but....
The drives are not hot swappable because they are in a programmed RAID, not a hardware RAID. However, once the new hard-drive is in place I though you could bring the system back online, and with it still running, it would sync the data. So the down-time would only need to be the length of time needed to place the new drive.
I maybe wrong about this though.
Also, our standard practice is to back up the system, and replace both hard-drives if something like this happens. We then restore the system on two the new drives. This way the drives are always the same age. The thought has been in our shop that if one drive goes, the other could go at any time because of their age and frequency of use.