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From: sonny@charybdis.harris-atd.com (Bob Davis)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: What is difference between Unformatted and Formatted HD capacity?
Message-ID: <5994@trantor.harris-atd.com>
Date: 4 Apr 91 21:46:17 GMT
References: <1991Apr4.203413.7550@news.larc.nasa.gov>
Sender: news@trantor.harris-atd.com
Reply-To: sonny@trantor.harris-atd.com (Bob Davis)
Organization: Advanced Technology Dept., Harris ESS, Melbourne, FL
Lines: 29


	When I compute drive capacity by the formula:

	Capacity (in bytes) = Cyls X Hds X Sectors X 512

have I computed "Formatted Capacity" or have I computed "Unformatted
Capacity"? Seems like on all the drives I have used, this formula
has given me the actual number of bytes available for my use -- which
I think must be called "Formatted Capacity".

	Precisely what additional data is recorded on the media that
is "overhead" and chews up some of the "Unformatted Capacity"?

	I have seen some drives having voice coil head positioning
mechanisms that have an odd number of heads. But certainly all drives
have an even number of platter surfaces. Is the missing head's
platter and its bytes counted as part of the "Unformatted Capacity"?

	Why is there a missing head on such drives? Is that head
*really* there and it is just that the platter surface is used for 
some overhead function? If so, what?

Thanks.
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