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First of all the drive is not tested, so compatibility is not guaranteed. I noticed that you're using laptop 2.5" HDD, which is not recommended in server environment. Enterprise class HDDs are highly recommended. Please check the Tested memory and hardware list. For more information, see Enterprise-class versus desktop-class hard drives comparison white paper.
I also noticed that ST9500423AS is a so-called 4K sector drive, while IT/IR RAID requires 512 bype. Although the drive can work at 512 byte emulation mode, I'm not sure if it's the cause of incompatibility.
Last Ubuntu is also not tested/supported, but you may want to try to configure the drive using the LSI MPT SAS BIOS Configuration Utility, by pressing Ctrl-C during POST. See if you can add the drive as a hot spare. See the Intel� RAID software users guide Chapter 6 for details.
Thanks for the response Edward. I did not use the Bios facility to attempt to fix the array because that requires powering down the server. Eventually I did reboot the server, without going into the BIOS configuration utility and the RAID magically fixed itself and incorporated the new drive into the Raid. It is now running optimally according to the mpt-status.
I understand what you are saying about the enterprise drives but these are simply too expensive for me, and there have been several independent studies that highlight that enterprise drives are not more reliable than commodity drives. (See Hard Disk MTBF: Flap or Farce? article in eWeek and other studies by Google etc). I am more than prepared to risk the occasional drive dropping out when I can replace it for a fifth of the cost of the equivalent size enterprise drive)
Normally I would replace the drives with a like size - but commodity drives as small as 320GB are hard to find these days! MTBF for laptop drives also take into account the fact that they are constantly being power cycled and take a few knocks being carried around. My own personal experience is that they are not as reliable or as fast as the enterprise drives in an 8 disk raid - but only by a very small margin, and that performance drop saves me £1600 per server over a 3 year lifecycle in disk costs, and also use less power.