I have a HP Proliant DL380 G5 server with P400 Array Controller. Right now it has 3 SAS HDD's 147GB each configured as RAID5. I am running out of space. Is it possible to install 4 new 300GB HDD's to form another RAID 5 to get 900GB additional space while keeping the existing 3x147 alive?
I don't know that array controller specifically, but assuming you have the physical ability to attach all of the drives, then yes you should be able to create a separate, 2nd RAID 5 array, without impacting the existing array. Back up everything ahead of time, of course.