Removing and replacing hard disk

G man wrote:
Raid-ar is showng disk 1 as dead, is that normal?

Yes

G man wrote:
When I get the hard drive back and if my original data is still on the hard drive. Will it sync automatically if i have added data to disk 2?

Hot-add (add while NAS is on) and it will be wiped and added to the array.

G man wrote:
Should I format the hard drive before I plug it back in?

Don't format it. Delete the partitions on the disk.

G man - if you returned the disk for a warranty replacement, it is unlikely that you will get your disk back. Instead you will get a refurbished disk that has no data and no partitions and the hour counter has been reset to 0. I have never gotten a Hitachi refurbished drive, but with Seagate the label has a green border and plainly states it is a refurbished drive. Just hot add it as mdgm has said and let the system do it's thing.

While it is gone, you may want to update you backup (which of course you have) to make sure that at least all your critical/important data (financial records, photos, etc) are backed up if not all you data. This is just in case you encounter a problem during the resync process. The resync process puts a lot of strain on the drives since it is constant file access for 6 or more hours, and if the other drive is showing any weakness it could fail during this time. While not common, this has been know to happen and then you have the dreaded dual HD failure. It is always better to have a backup and not need it than the reverse.