CD-Rom is a directly attached storage device that is written and read sequentially.
Direct access storage devices (DASD) seek to a particular cylinder and sector, as directed by the controller or application, and start reading or writing from that particular address. The line here is blurring a bit since although the data is still written sequentially, newer CD re-writable technology has enabled optical CD media to be written more than once. This means (under some older formats)the CD-drive will need to "seek" to the next available free space on the optical disk before writing a new session (like when your local film developer can put multiple digital picture sessions on a single CD).
The term DASD is normally used to describe mainframe storage devices. You can always tell an old mainframer from an "open systems" storage person by the terms they use to refer to disk storage. As soon as they say "DAZ DEE", you should bow in respect since they have most likely "been around the block" as they say.
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This was first published in March 2003