Upgrading from a 10MB, 20MB, or 40MB to a 67MB drive requires
a 3B1 power supply and a 3B1 case top (the 40MB and 67MB drive is
full-height).  There are other solutions to this -- you can get
half-height drives that have seek times faster than the 80ms of the
old 10MB and 20MB drives, and which have more disk space.  So long as
the drive has a ST506/MFM interface, has less than or equal to 1024
cylinders and less than or equal to 8 heads, the drive will work
without *any* hardware modifications.

	To upgrade to a disk with more than 8 heads and more than 1024
cylinders there are several approaches one can take.  Gaining more
cylinders is the easiest of upgrades: all you need to do is replace
the WD1010 disk controller (which is socketed on the motherboard at
location 21H) with the pin-for-pin compatible disk controller, WD2010.
The WD2010 comes in several varieties, and all seem to work for the
most part (WD2010A, WD2010B, etc.).  The WD2010 is a very difficult
part to come by -- Intel makes (or made) a pin-compatible version of
the WD2010, the Intel 82064.  There has been some discussion on Usenet
that people haven't been able to get the WD2010 to work in their older
revision motherboard UNIX PC or PC7300.  These machines have a
daughterboard that handles the disk circuitry, instead of the
all-in-one chip that was replaced in the later models.

	The operating system, from at least release 3.0, supported an
unreleased motherboard revision, P5.1.  The P5.1 revision level (like
the P3...P5 that you see during the boot phase) includes some extra
features such as an extra disk head select (expanding to 16 read/write
disk heads) and extra drive select (allowing expansion to two
simultaneous hard disks).  With the appropriate hardware modifications
(all requiring some expertise in soldering and reading schematics),
one could upgrade his motherboard to emulate this undocumented
motherboard revision.  These upgrades weren't released to the public
by either CT or AT&T during the life-cycle of the product, but were
later released and made public by several people in several different
forms.

    o   John Milton <jbm@megalinx.net> has a prebuilt circuit board that 
	offers up to 4 hard disks and 2 floppy drives, but be forewarned 
	that the operating system only supports the two hard disks and 
	one floppy drive.  If the operating system patches could be made, 
	John's hardware would support it.  He's offering a prebuilt and 
	pretested board that can be wired into the motherboard.
        The motherboard wiring (jumpers) and soldering will have to be done 
        as well before you can use John's board (this is not a plug in and
        go situation -- it requires some time to wire).  The board and
        instructions currently cost $75, but contact John for pricing.
	[As far as I know this is no longer available from John]

    o   SSDL/ICUS Hard Disk Upgrade Version 2.0.
	Gil Kloepfer, Jr (gil@limbic.ssdl.com) is currently (10Jan92)
	offering the second version of the popular ICUS do-it-yourself
	hard-disk upgrade kit.  From the announcement, answering the
	question "What is it?" --
	"It is a single-chip upgrade to the 3B1 that allows a second
	hard disk to be added and a 4th head select line to allow
	disks with more than 8 heads to be used.  It is a superset of
	the functionality of the P5.1 PAL (ie. you don't need P5.1 to
	use the V2.0 PAL), and completely emulates all the
	functionality of the earlier ICUS V1.0 upgrade.

	"This upgrade *DOES NOT* extend the number of cylinders
	(>1024).  You must purchase a WD2010 to replace the WD1010
	chip on the motherboard if you wish to do this."

	Full information about availability and pricing can be
	obtained from v2upgrade@limbic.ssdl.com.
	[This is also no longer available from Gil *See Below*]

	[The following was received from Gil]
	Just letting you know that the WWW page with the HD upgrade 
	instructions and the PAL equations is up and operational.  My 
	apologies in advance that the formatting is basic and the 
	diagrams are scanned-in and put up as GIF images.  I'm sure 
	those who need the info will be grateful anyhow.

	The URL is:  http://www.gc2.kloepfer.org/~gil/hdinstr.html

	There is no other pointer anywhere around to this location, so 
	if there is some kind of 3B1 web site, it should likely be 
	referenced there.

	Enjoy!  If you see any glaring mistakes, send an email to
	"Gil Kloepfer Jr." <gil@gc2.kloepfer.org>.

    o   FIELD P5.1 PAL upgrade.  The P5.1 instructions were posted to 
        unix-pc.general a long time ago, and are now archived on OSU in
        the P5.1.Z file.  This requires a preprogrammed PAL chip to be made.

    The largest disk which can be handled by the UNIX PC/3B1 is:

    o   Motherboard revision P3...P5 (WD1010 disk controller)
	8 heads x 1024 cyls x 16 sectors/track x 512 bytes/sector  =  67.1MB

    o   Motherboard revision P3...P5 (WD2010 disk controller)
	8 heads x 1400 cyls x 16 sectors/track x 512 bytes/sector  =  91.7MB

    o   Motherboard revision P5.1 (modified) (WD1010 disk controller) 
	16 heads x 1024 cyls x 16 sectors/track x 512 bytes/sector = 134.2MB

    o   Motherboard revision P5.1 (modified) (WD2010 disk controller)
	16 heads x 1400 cyls x 16 sectors/track x 512 bytes/sector = 183.5MB

	NOTE: 1400 cylinders is the #define HDMAXCYL in
/usr/include/sys/gdisk.h -- although the WD2010 can support up to 2048
cylinders, the operating system cannot.  Also, with the multiple hard
disk upgrades you can have two disks that can be as large as the above
sizes for the P5.1 modified motherboard revision.

	FINAL NOTE ON THE WD2010: Some folks have reported troubles
with their systems after installing the WD2010, far too many to
discount as due to bad chips.  Thad Floryan was irritated enough by
this to take time away from sheep-herding and solve the problem.  This
problem occurs only on certain versions of the 3b1 motherboard.
Short and sweet, quoting from Thad here:

	"So, in overview, the complete and correct "DRUN patch" modification 
	to a 3B1 motherboard which does function with a WD1010 but does not 
	function with a WD2010 is:
		1. separate and lift [13N] 74123's pins 1 and 2 
		   from the motherboard
		2. run a wire from the lifted [13N] pin 1 
		   to [13M] 74F10 pin 7 (ground)
		3. run a wire from the lifted [13N] pin 2 
		   to [13K] 26LS32 pin 3
		4. replace R63 per:
			original:  6.81K,  1%, 1/4W
			new value: 4.75K, 1%, 1/4W
	Parts list:
		1. new 74123 (reason for this is described below)
		2. 4.75K, 1%, 1/4 W precision resistor
		3. less than one foot of 30ga "wire-wrap" wire 
		   for the two patches"

	And additionally:

	"If your system is one that DOESN'T have the "DRUN Patch"
	then putting the WD2010 in your system will cause you a LOT
	of grief.  From my observations on MANY systems, it's not
	always obvious whether the patch exists on one's system;
	some factory runs implemented the patch along the lines
	of what I described in my posting last December [excerpted
	above], and some runs had the patch integrated into
	(presumably) new motherboard layouts where the legs of the
	74123 chip are NOT sticking up in the air.  If the resistor at
	R63 has the color code bands, then the presence of 4.75K 1%
	means the patch is already applied (the value of 6.81K 1%
	means you have the old data separator circuit which will NOT
	function properly with a WD2010); if the resistor is the RN05
	type (no color bands) then you probably won't be able to read
	the value and you'll either have to remove it (to read the
	value) or forget the whole thing.

	The ABSENCE of a capacitor at C252 is also a good clue one's
	motherboard has the DRUN patch already applied."