A double capacity adapter circuit is connected between host equipment and one or more magnetic disk drives in order to adapt the host equipment, which has been programmed for use with relatively small capacity magnetic disks, for use with double capacity magnetic disks in the disk drives coupled thereto. As originally programmed, the host equipment formats one side of a small capacity magnetic disk with first format data in response to an index pulse from the associated disk drive, and the other side of the small capacity magnetic disk with second format data in response to another index pulse. In order to format the double capacity magnetic disks with the same program, the adapter circuit delays the index pulses by a time corresponding to half a revolution of the magnetic disk, thereby producing pseudo index pulses. The adapter circuit further enables the host equipment to cause each double capacity magnetic disk drive to write the first format data on a first half of one side of the double capacity magnetic disk in response to one true index pulse, and the second format data on a second half of the same side in response to one pseudo index pulse. Thus, for example, the complete format data that have been programmed to be written on four double sided small capacity disks can be written on two double sided double capacity disks.