These are actually the first external drives TI used, before the switched to the MPI Model 51s. And, as already noted, they could be connected to the sidecar disk controller box (part of the original peripheral train: 32K, RS-232, Video Controller, P-Code, and Disk Controller). I have a whole set of the peripherals, but I do not have one of this style drive. I had a friend with a pair of them though, ISTR they made a humongous chunking sound when they opened.