>Hello Al and thanks, my drives appear to be sitting in a caddy so adding new ones should be a case of just sliding them in, I also have ACU installed, in dummies terms ( I.e me ) can I just physically add two more drives and reboot, then run ACU to configure them ?, I have been on HP's site but I glaze over after a few minutes reading as I don't have the knowledge to digest it ( which is why I like sites like this one ), from what I've read it seems that RAID dumbs down to the smallest and slowest drive in the set, so as long as I add two identical drives ( capacity and speed ) I should be OK ?

If you mount the drives in caddies (sometimes called sleds) then slide and lock them into the server from the front, you probably have a hot-swap backplane. The act of sliding them in connects their data and power lines, you don't have to worry about internal cabling.

Yes, use two identical drives. SAS is preferred (although the most expensive). You can also attach SATA drives to an SAS controller and save some money. If you do so be absolutely sure to use only enterprise SATA drives such as Western Digital RE4. These support Time Limited Error Recovery (TLER). Consumer drives, even high performance ones such as WD Black do not support TLER.

I found this out the hard way, I was called in to support an SBS 2011 Standard server, 4 drives in RAID5. Exchange was periodically reporting performance problems (long delays in some write operations) which is a reliable sign of hardware problems. Drives dropped out of the array about every 4 months on average. I installed the ACU, which can show you exact model numbers. I looked them up and found they were all WD Black. Gave the bad news to the client, he had some words with the server supplier. We replaced the Blacks with RE4s one at a time, allowing the array to rebuild after each replacement. No more problems.