can a mac pro mid 2012 support the use of 4TB hard drives in each hard drive bay??

Yes, you can.

Some of the latest WD drives are adding an additional platter. To do this in the same overall height drive, they are taking back the mounting hole screw depth of the screws directly adjacent to the platters.

This may make these mounting screws on the Mac Pro sleds too long. They are SAE (American thread) #6 screws, 32 threads/in.

i've got two WD black caviar 4 TB drives in my 2012 5,1 and they seem to work fine.

OK. looks like the 6 TB drive may need a new sled to fit...

OWC sells such a sled. Link in my other post on using 6TB drive.

hi hatter.

just bought one based upon that post.

thanks /very/ much for the heads up and the link.

cheers

OWC Hard Drive Bracket (6 holes) for Mac Pro (2009-2012)