How To Get Notified When Hard Drives Fail In Dell PowerEdge Servers

Yeah, it sounds like 0 and 1 are RAID 1 (mirror) and 2 and 3 are also RAID 1. As for disk 4, take a look here. That is the manual for your 2850 and shows you what all the various blinking scenarios on the disks mean.

You mention you don't see the second RAID set in Windows. Are you just looking at Windows Explorer? If yes, that's normal. You need to launch Server Manager, navigate to Disk Management and there you should see the 146GB drive. Right-click it to initialize and format the disk and assign it a letter. Once it finishes formatting, you'll see it in Explorer just like the 73GB mirror.

I did this recently between two identical HP G5 servers without problems. Hardware-based RAID-controllers who arent stupid saves their logical volume configuration on the disks, so it should not be a problem if you have the same RAID controllers (at the same firmware version).

You should however make sure you have a very recent backup/ghost image of the logical volume before doing this.