HP Smart Arrray 5i Issue

I have a HP DL380 G2 server with 36Gb drives in it configured as Raid 5 with an online spare. I recently bought 72Gb drives to replace the 36Gb drives. I know the drives are hot swappable so I started to replace the drives and everything was going fine until I got to the 4th out of 6 drives. When I put the 4th drive in it looked like it was starting to rebuild but then it failed and now all I get is “Ready For Rebuild”. I have rebooted the server tried different drives and even tried putting the 36Gb back in. I am a little stuck and I am worried if I have a drive failure before I get this fixed. Any help would be great!

Thanks!