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.
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