Correct, Dell purchases the drive with a custom firmware image.  That info is what the firmware is looking for.   An EOM drive does not have that so it's rejected. 
We have a PS5000e that went out of warranty a couple of years ago, and I've set it up to host scratch volumes for our terminal servers. One of the drives failed, so we replaced with the same Hitachi model taht we bought from another vendor because it was no longer available from Dell. Now we're getting the "One or more drives in array are not approved" message.  We should be able to return the drive if we have to, and I guess I would need to contact Dell to find out what kind of replacement drive we can order. The drive appears to be healthy, but it's a hot spare. Can I expect it to work properly, or am I out of luck here? 
Drive not approved for array