07 March 2017
I have a Pegasus R4, too, and I have had problems in the past with it not mounting.
This is going to sound strange -- and I haven't tested this thoroughly -- but I think the R4 Thunderbolt interface might be susceptible to RFI (radio frequency interference).
If you are next to your Mac with your cell phone, turn it off, or move it to another room.
Unless it was pure coincidence, I seemed to have better luck with the R4 mounting when my cell phone was not in the proximity of the Mac (and more specifically, Thunderbolt).
I wrapped aluminum foil around both hardware ends of the Thunderbolt cable for an extra measure of RF protection.
I did NOT, however, make a tin foil hat. :D