6.2 Drive Types

As distinct from diskette types and formats, six drive types have been installed in PC-compatible systems:

5.25" 160/180 KB (SSDD)

Single-Sided, Double-Density (SSDD) was the standard FDD in very early PC-class systems. These drives read and write only SSDD diskettes.

5.25" 320/360 KB (DSDD)

Double-Sided, Double-Density (DSDD) was the standard FDD in PC-class systems, and 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.

5.25" 1.2 MB (HD)

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 often found as a second FDD in early systems with 3.5" primary FDDs. These drives read and write any 5.25" format. A diskette previously formatted or written to by a 5.25" DD drive and then written to by a 5.25" HD drive may not subsequently be reliably readable in any 5.25" DD drive.

3.5" 720 KB (DD)

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.

3.5" 1.44 MB (HD)

High-Density (HD) is the standard FDD on mainstream systems for the past decade. These drives read, write, and format any 3.5" HD or DD diskette.

3.5" 2.88 MB (ED)

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" diskette in any format.

It's worth noting that 5.25" drives and 3.5" drives use different methods for write-protecting diskettes. 5.25" diskettes have a write-enable notch. To write-protect a 5.25" diskette, cover that notch with opaque tape. 3.5" diskettes have a write-protect hole with a sliding shutter. To write-protect a 3.5" diskette, slide the shutter to uncover the hole.