Since colors do not match what should be ATX power supply wires, I
suspect this is an AT type machine. Is this a 486 computer or an
computer not using PCI cards?

One voltage is wrong because I believe it should have read -5 volts
and not +5 volts. The + 12 and -12 volts are both too low. Normally
this would means doing the next step for excessive ripple voltage
(which requires better equipment). However + and - 12 volts don't
drive anything critical on AT type computers. This post will continue
on an assumption it is an AT type computer - noting that if it is an
ATX computer, then it might be a severely questionable - probably
defective - power supply. But we assume it is AT.

Anything that is IBM original checks out just fine with those
diagnostics from IBM. Rarely do diagnostics get updated when they are
working fine - Y2K not really considered important.

But for added peripherals such as CD-rom or hard drive, then you must
download the diagnostics from the manufacturer of that peripheral.

A 30 gig drive in an older computer like this means the computer is
probably using some type of Bios Extender - Ez-Bios or whatever that
hard drive manufacturer provided. You must identify that Bios Extender
and obtain operational details. For example, some require the BIOS to
be set to a non-standard setting. If you did as so many others want -
fix this and fix that rather than first collect facts - then it is
possible you have lost what is the unique settings for that Bios
Extender program (that is loaded on the hard drive).

Generally Bios Extender programs would announce themselves when
computer first booted normally. Software is often provided by hard
drive manufacturer on manufacturer's web site. But at any rate,
discover if and what that Bios Extender is. Normally your computer
(check the manual) would only understand hard drives of maybe 8 Gig
max. To see a 30 Gig drive, unique software such as a Bios Extender,
or an ISA slot card that provides extended Bios, or a hard drive
interface card with that Bios Extender must be somewhere in that
machine.

Diagnostics from hard drive manufacturer might help to identify that
Bios Extender.

Also what can provide useful information is to boot machine with DOS
(if possible) and run the MS provided program called FDISK. Don't do
anything that would fix or modify the drive. Just select the option
that would read drive information ... to learn how that drive is setup.
And yes, this assumes you can boot DOS.

BTW, when we are all done, you want to record these unique setting on
Post-It notes and mount those notes inside the machine.

Now if I understand your original post - which I never really saw -
you can boot from floppy but not from hard drive? I was never sure
what did boot and how failed booting failed. Which OS does and does
not boot how? If this is a Windows 95 machine, then it does not see
the 30 Gig hard drive. Windows 95 is limited to hard drive of only 2
Gigs (if I remember correctly). So you can see how I find some of your
information contradictary.

At any rate, barring a missing negative sign and assuming this is an
AT type machine, the power supply would be sufficient meaning we move
on to other suspects. I am very suspicious of how that hard drive is
configured and suspect a Bios Extender - which sometimes makes
complexity.