What Does High Density (HAD) Mean?
"High density" in IT can be used in various ways. One way is in describing a "high-density disk." The high-density disk is a somewhat obsolete idea in IT, since newer storage technologies have led engineers and others to use "area density" or "surface density" as a metric, rather than labeling storage media as high density.
Techopedia Explains High Density (HAD)
In the era of the floppy disk, a high-density disk was one that held more data than the average disk. Typically, a high-density 5 ¼" PC disk held just over one megabyte of data. A high-density 3 ½" disk held nearly 1.5 MB.
Generally, floppy disks have been eclipsed by new forms of storage. One is the emergence of USB connected flash drives and similar storage media. These drives now have storage density well above 1 GB per square inch. In fact, it is not unusual for a thumb-sized drive to have a storage capacity of dozens of gigabytes. This is one reason why the term "high density disk" has been largely phased out.