Can I Mount a Hard Drive in Mac Pro Tower as Read Only?
I use a program called Disk Arbitrator. Its used to block auto-mounts when you connect devices and can also be used to auto mount drives in read-only instead of read.



(Disk Arbitrator is the green icon, far left)

From my understanding it hops in before the system can mount the drive, and instead mounts as read-only. Probably not as good as using a hardware block, but in most cases its sufficient (and I rely on it for similar purposes)