You should have a configuration/management interface or CLI commands to configure and/or check the status of your RAID system, the arrays, logical volumes and the hard discs.
Could it be that the fourth disc is configured as a hot standby disc and that is the reason why it is off?
Which operating system are you running?
Can you access logical volumes on the RAID?
You may have a spare (replacement) drive whose size in sectors is slightly less than the original drive.
What operating system are you using? We can look at the hpacucli output to determine this for sure.
I will update this answer once we get more information.
Edit:
See my answer on: Smart Array P400i Physical Drive failed after being replaced for the tool download link for Windows 2008...
I'm specifically looking for the output of ctrl all show config.
Edit:
Now that I've seen your hpacucli output, you're in an incredibly rare situation.
logicaldrive 1 (68.5 GB, RAID 1, Interim Recovery Mode)
physicaldrive 2I:1:1 (port 2I:box 1:bay 1, SAS, 73.5 GB, OK)
physicaldrive 2I:1:2 (port 2I:box 1:bay 2, SAS, 72 GB, Failed
Your original 72GB drives were actually larger than 72GB. As you can see, your healthy disk is a 73.5GB disk. HP changes disk manufacturers during the product lifetime, keeping the same spare part numbers. In this case, the replacement sent to you is a true 72GB drive. The rebuild would definitely not work.
The only thing that can fix this one is an equal or larger-size physical drive. Simply send this output to HP and ask for a 146GB disk if you're under warranty. If you're not under warranty, just get a 146GB disk and it'll rebuild as a "68.5 GB" RAID-1 member.
Also, ask for a 10k RPM 146GB disk. Your old 72GB disk is a 10k RPM (which was discontinued in that capacity LONG ago). The HP replacement is a 15k RPM drive.
Best Answer
I'm going to suggest that you try this with an HP Smart Array controller. HP went out of their way to develop their SmartDrive technology and new drive carriers for the ProLiant Gen8 product line.
See: http://www.youtube.com/watch?v=s1hOYjqRvSQ
Other controllers simply wouldn't be aware of the new drive indicators. Note the additional electrical contacts on the backplane side of the drive tray? Your LSI RAID controller would not be able to interface with those.