Odd RAID ...

[#1]

I've got  a friends HP PC here, hard drive is failing big time. I look at the disk and its capacity is about 135GB with a 10GB recovery partition.
I take the PC apart to get the disk out and find its a 400GB drive. When I dig into the BIOS the SATA controller is set to RAID.
So with a single disk does this mean its created a Raid set on a single disk, thus reducing the capacity or what?

[#2]

You can't create a raid on a single drive, 2 is the minimum.
When you looked using what? Sounds like a 3 partition setup with the 3rd empty space.
You used to be able to set to drives to smaller capacity in the BIOS to fool older BIOS's but that was a lot of years ago, most these days just auto detect. Or was there something like the Seagate tool to help older BIOS's on it? Shows when booting usually after the BIOS but before the OS loads.
400Gb is a strange size as well, 300-320Gb were common then 500Gb but 400..

[#3]

I know its all very odd. Vista doesn't show there being any free space on the drive (I've got it plugged in on a spare SATA cable to take all the user data over).  I could dig round in the Disk partitioner and see if there is a pile of unused space but given the noises its making its all rather moot!
Its a Samsung  HD400LJ which is a 400GB drive - which as you say isn't very comment.
Just seems odd that HP would ship a 1 disk machine with the SATA controller set to raid.