DEC's RX-02 dual eight inch dual floppy drive was commonly applied to both UNIBUS and Q-BUS pdp-11 systems. In high density mode, the RX-02 is capable of storing 512 kb of data; in single density, 256 kb.
The UNIBUS controller for the RX-02 is the RX-21, for the Q-BUS, the RXV-21. Both are NPR devices capable of transferring data to/from the device without processor intervention (other than the initial setup obviously). The different bus devices are software compatible.
The disk itself is a single-sided eight inch plastic disk (coated with magnetic material) enclosed in a protective envelope. The envelope is open on both sides in the head access area. The Read/Write head accesses the disk from below, while the disk is held in contact with the head from above by a felt pad on a solenoid-controlled arm.
The drives proper appear to be of DEC manufacture (many devices used the ubiquitous Shugart Associates drives) and use a custom device controller board common to the two drives.
The RX-02 device here is in use with both the
systems. Disk longevity seems to be good; I've got several disks that
are better than a decade old (with original data) that read/write