HP ProLiant ML350 G6 Server
Solution 1:
The HP ProLiant ML350 G6 is what we use at work. The RAID is configured automatically by the RAID controller card when the machine is first switched on and the level of RAID is determined by the number of disks. So with 4 disks you get RAID 5 with two you get RAID 1. It states in the HP documentation the RAID level that will be applied to your drives (pg. 93).
http://bizsupport1.austin.hp.com/bc/docs/support/SupportManual/c01711832/c01711832.pdf
You are only seeing one HDD as RAID has configured the two disks into a logical one for you automatically. If you need to alter the settings you'll need to use the RAID utility and re-install the OS.
Update Edited to include updated info on default RAID levels. I was wrong and sloppy and didn't check my facts; ewwhite was correct.
Solution 2:
Your server should have had a version of HP's 'Proliant Support Pack' installed on it, it's basically a bunch of hardware specific drivers and tools that get installed as one big install. You could check if this has been installed via Control Panel and the Installed Software bit.
If it's been installed then you should have a HP tool ready called 'HP Array Configuration Utility', it's a Windows tool that helps you setup and tune disk arrays.
Now yours is pretty straightforward setup and you should hopefully see the two disks, bound together into a single RAID 1 mirrored array and presented to the Operating System as a single 'logical disk'. If you can check that and it looks that way then you're doing fine, the hardware is doing the work and you're protected, it'll even alert you if there's a disk problem.
If it's not like that then come back and tell us how it's setup and we'll try to help ok.
Solution 3:
If you see a single disk the RAID is automatically build on first boot.
If you have installed HP Proliant Support Pack compatible with your model, you can use "Array Configuration Utility to see the status of the RAID"