Most IBM 1131 models included an internal 1 Mb removable-cartridge hard disk drive occasionally called the "Ramkit." The 1130 was the first IBM product to feature this, or any, easily removable single-platter disk.
The disk was organized as 203 cylinders, two surfaces, four sectors per surface, and 321 16-bit words per sector. Formatted for use by the Disk Monitor System, three cylinders were used for bad-block replacement, and one word per sector was used to hold a sector address, giving a formatted capacity of 512,000 words.
The read head assembly was moved by a voice coil actuator but positioning was controlled by a solenoid-driven ratchet mechanism. This mechanism resulted in track-to-track movement times comparable to a floppy disk drive, and produced a characteristic buzzing sound. Many experienced users recall how they could follow the progress of a Fortran compilation from the sounds made by the disk drive.
The IBM 2315 disk cartridge used by the 1130 should be familiar to most minicomputer users of the era, as its design was licensed by IBM to DEC, Hewlett-Packard, Wang, Xerox, Diablo, Bruker and many other computer manufacturers. While later drives generally used faster head positioning mechanisms and different sector layouts, the technology remained popular into the 1980's.
1131s with the Storage Access Channel option could be outfitted with additional disk drives. Options included single or double Model 2310 Ramkit drives, or the Model 2311 multiple-platter drive (pictured).