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Number of Read/Write Heads
Each hard disk platter has two surfaces, one on each side of the platter. More often than not, both surfaces of the platter are used for data on modern drives, but as described in the section discussing the number of platters in a drive, this is not always the case. There is normally one head for each surface used on the drive. In the case of a drive using dedicated servo, there will be one more head than there are data surfaces. Since most hard disks have one to four platters, most hard disks have between two and eight heads. Some larger drives can have 20 heads or more.
Only one head can read from or write to the hard disk at a given time. Special circuitry is used to control which head is active at any given time.
Warning: Most IDE/ATA
hard disks come with "setup parameters" intended for use when configuring the
disk in the BIOS setup program. Don't be fooled by these numbers, which sometimes bear
confusing names like "physical geometry" even though they are not actually the
physical geometry at all. For today's drives these numbers have nothing to do with
what is inside the hard disk itself. Most new IDE/ATA disks these days are set up as
having 16 heads, even though all have far fewer, and new drives over 8.4 GB are always
specified as having 16 heads per the ATA-4 specification. BIOS translation issues can make
this even more confusing. See here for more details.