DASD stands for Direct Access Storage Devices. Basically: Hard drives.
Sometimes referred by long time IBM'ers as "Hard Files".
An interesting aside here follows our industry's Jargon March of Progress.
In the 1970s, direct-access magnetic storage devices were called "Disk Packs." Engineers would reach into the bowels of the mainframe computer, retrieving and inserting these large stacks of spinning disks which together formed a virtual disk drive.
Disk packs had names (Volume IDs). One for payroll, one for accounting, one for student masters, etc.
By 1980, these gadgets were called "fixed disks," because they weren't being removed any more. Two-hundred fifty-six million characters of online storage? How could we ever use it all?
In 1981, IBMers tried to punch up the friendliness of the brand new microcomputer which was now rebadged, the IBM Personal Computer (PC). They came up with simple, friendly names for equipment and processes. Diskette drives became floppy drives; main storage and static RAM became memory. A fixed disk was now a "hard file."
In 1988, with the advent of the AS/400, we were treated to the acronym DASD.
By 1994, with AOL, the World Wide Web, and Windows 3.11, the phraseology changed again and "hard drive" is now the standard.
Oh, for the day...