As distinct from diskette types and formats, six drive types have been installed in PC-compatible systems:
Single-Sided, Double-Density (SSDD) was the standard FDD in very early PC-class systems. These drives read and write only SSDD diskettes.
Double-Sided, Double-Density (DSDD) was the standard FDD in PC-class systems, and was often found as a second FDD in early AT and 386 systems. These drives read and write single-sided (160/180 KB) and double-sided (320/360 KB) formats.
High-Density (HD)?this and all later formats are double-sided, but that part is no longer stated?is the standard FDD in 286, 386, and some early 486 systems, and is often found as a second FDD in early systems with 3.5-inch primary FDDs. These drives read and write any 5.25-inch format. A diskette previously formatted or written to by a 5.25-inch DD drive and then written to by a 5.25-inch HD drive may not subsequently be reliably readable in any 5.25-inch DD drive.
Double-Density (DD) is an interim standard, commonly found as a primary drive in early low-end 286 systems, and as a secondary drive in a few PC-class systems and many 286, 386, and 486 systems. These drives read and write only the 720 KB DD format.
High-Density (HD) is the standard FDD on mainstream systems for the past decade. These drives read, write, and format any 3.5-inch HD or DD diskette.
Extra-Density (ED) is a failed standard, introduced by IBM and now effectively obsolete. ED diskettes are very expensive?typically $3 each versus $0.25 for a 1.44 MB diskette?which doomed the format. These drives are difficult to find new nowadays, but can read, write, and format any 3.5-inch diskette in any format.