I would like to upgrade my DL185 G5 to take 3tb hard drives. the P400 controller does not support 3Tb hard drives. If I upgrade the controller to a P212 (which I already have), Do I simply use a SFF-8484 to SFF-8087 cable. Sorry I don't understand the backplane does it have it's own SAS expander built in.
Yep, the backplane has its own expander in some models. You did not specify which HP ProLiant DL185 G5 model you have, though. How many drive bays does your unit have? That dictates the backplane setup. 12-bay units have a built-in expander, for instance.
I really wouldn't try to stuff newer drives into an old unit like that, given that the DL180 G6 model is relatively affordable and more capable.
But using the right cables will work. You'll need the older SFF-8484 cables on the backplane side and the newer SFF-8087 on the controller side.
BTW - Do you have cache for your P212?