Hi John,
Actually, whether you need to power off the System or not depends on your RAID controller and the removable drive rack you are using. On a RAID card that supports this function (I actually have not checked with regards to the Highpoint though - usually high end controllers will support this), if you have suitable IDE removable racks - with keyed power lock functions for example, you could actually remove a failed drive and replace it without the need to power down the server. And again, as long as the card supports it, the card could detect the drive replacement and automatically begin a rebuild process on the new drive - all without needing to power down the server. Remember, as far as the server is concerned, there is only ever one drive connected to it. The hardware RAID controller hides the fact that there is more than 1 drive present. This option does NOT exist for a software RAID solution.
If you use dissimilar drives in your RAID pack, the smallest drive in the pack determines what the RAID pack becomes. However, you should NEVER replace a failed drive with one of a smaller capacity.
Cheers,
Kelvin