|Presumably you have the 4 disks in a RAID array? (RAID 5 I'd guess.) There's no problem with using different sized drives in the other two slots, but you won't be able to add them to the array (well, you can but you'll only be able to use 36GB on each disk). But you can pair them in a separate RAID 0 or RAID 1 array or use them as individual drives.|
I have actually upgraded the disks on a X Series with a RAID 5 array by replacing a 36GB one with a 72GB one, letting the array rebuild and then repeating the process until all were replaced. It takes a while, but it works.