rpiboot does not work on the Mac for BCM2835, due to an bug in the usb-controller of the BCM2835.
This affects the original CM. The bug is fixed on the BCM2837, so it should work with the CM3.
Note There is a bug in the BCM2835 bootloader which returns a slightly incorrect USB packet to the host.
Most USB hosts seem to ignore this benign bug and work fine; we do, however, see some USB ports that
don't work due to this bug. We don't quite understand why some ports fail, as it doesn't seem to be
correlated with whether they are USB2 or USB3 (we have seen both types working), but it's likely to be
specific to the host controller and driver. This bug has been fixed in BCM2837.
I just build rpiboot on my MAC and used it to make the eMMC of the CM3 available as an 3.9Gb
external drive. Works fine.