External storage device, control method for computer system, and control method for external storage device

The present invention provides an external storage device, a control method for a computer system, and a control method for the external storage device. When the external storage device having a plurality of storage units is connected to the computer system, operation complication that drive directory trees and icons of each storage unit are displayed in a display device can be reduced. When a hard disk drive 200 is initially connected, the CPU 110 of a personal computer 100 assigns respective drive letters to a virtual CD-ROM drive area 222 and a hard disk drive 224 in the hard disk drive 200, and records information relating the drive letter assignments as drive letter information in the registry. The CPU 110 then executes auxiliary application software stored in the virtual CD-ROM drive area 222 to delete and disable assignment of a drive letter to the virtual CD-ROM drive area.