Definition - What does Double-density (DD) mean?
Double density refers to the storage capacity of floppy disks for PCs. 5.25 inch double density disks hold 360KB of data, while 3.5 inch double density disks hold twice that amount reaching up to 720KB of data.
Techopedia explains Double-density (DD)
PC floppy disks are now largely obsolete, but where they are available, they may come in double density storage sizes. Newer devices lack floppy disk drives, and USB-connected drives have largely replaced these external data storage units.
Today’s storage media often have much higher storage capacity designations. Small thumb drives and other types of USB-connected drives typically have dozens of gigabytes of storage. The storage capacity of extremely small drives is now nearing the terabyte range. This has led to a shift in how tech companies and other parties label disk or drive storage density.