Question : Problem: Why did my hard drive crash

I have windows 2003 server, with an INtel 631x/ESB RAID controller. It is set up for Mirroring, with two Seagate 1TB SATA drives. I have three questions:
1) Is there any reason this drive would have crashed within 3 months of being installed on a brand new server?
2) Why is the rebuild causing the server to reboot?

3) Can I install another 1TB SATA drive in this array to replace the failed one or does it have to be a SEAGATE?

Answer : Problem: Why did my hard drive crash

1.  Based on two in your second comment, I would run a full diagnostic on the drives using the hard drive maker's diagnostic software.  It could be that there's a bad sector - or worse - that threw the RAID offline.  But again, it could just have been a bad drive/the drive's time to go.
2. See #1 in this comment.  But that still doesn't say it's the RAID.  An unexplained shutdown is just that - an unexplained shutdown.
3. Hot-Swap simply means you can replace it while it's running.  There's always the possibility it's a horrible implementation of RAID, but frankly, I've never heard of a system you couldn't rebuild (assuming the (relpacement and otherwise) drives and controller were functioning normaly)