IBM RT PC Options Installation page 105

6. Pull the wire latch aside, place the diskette drive on the rails,
and slide it into drive position B. Ensure that the wire latches
hold the drive firmly in place.
7. Look at the diagram of the disk and diskette cable bundle, which
is already installed. The first two (if two are installed) fixed-disk
drives and the diskette drive in position A connect to this cable
bundle.
Section 2. Removing and Installing Drives in IBM 6150 2-61