From the firmware version, I am assuming this is indeed an HP drive you are trying to install.  I would strongly recommend against this, as its firmware is not guaranteed compatible with the controller, which may or may not cause you problems, now or in the future.  As it is, it looks like you already have a mix of generic retail and Certified drives in there.  That said, and assuming you want to proceed anyway and that the incompatibility is not causing it, it could be several things.  What are the makes and model numbers of each drive you are using (and sizes, etc.)?  Your HP drive is labeled HP, but who is the actual mfg?  Drive construction can vary a bit between manufacturers, and because of this, the final available size can vary slightly.  For example, one drive might be 146.3GB and another might be 146.5GB.  If your existing drives are even 1MB larger than the HP, the controller will not see the HP as a suitable drive.

In OMSA, go to Storage, PERC, Information/Configuration (link at top of page), and choose Create/New VD from the dropdown menu.  Does your HP drive show up there?  If so, then there is something about the spec of that drive (probably size) that the controller deems as unsuitable to be used in the RAID 5 with the other disks.  If the drive doesn't show up in the Create/New wizard, then I would assume a drive problem or general incompatibility of the drive to communicate with the controller.

Yes, the HP drive shows up when I click on Create Virtual Disk: 

Also does in a RAID 5, all the disks must be the exact same?

Here is why (notice the capacities)

HP SCSI disks are bad... :(

Thanks!

:)  No problem

Drives do not have to all be the same (again, you already have two differing drives in your RAID 5 - drive 2 is different from drives 3 and 4), BUT the new drive must be at least as large as the other drives in the RAID 5 (I suspect it is some small - but significant - amount that is keeping it from being used in the RAID 5).  If this 73GB drive does not work, then you could try other 73GB drives to find one whose variation is within the size range of your other drives - OR use a 146GB drive.

68.24 vs. 67.74. How embarrassed it is!  :P

dodo ... there's nothing to be embarrassed about.  They are both technically 73GB hard drives - we shouldn't need to look for nuances like that.

Yes. Sorry for this joke. 1932. You are a RAID expert. I can hardly find a post you missed. :)

Sorry ... I didn't catch that it was a joke :)