You might try a boot floppy so you can add the driver. For a floppy to
successfully boot Windows NT the disk must contain the "NT" boot sector.
Format a diskette (on an NT machine, not a DOS/Win9x, so the NT boot sector
gets written to the floppy), and copy ntldr, ntdetect.com, and boot.ini to
it; Edit the boot.ini to give it a correct ARC path for the machine you wish
to boot.

In your case you'll want to change the arc path in boot.ini from multi
syntax to scsi syntax to indicate that Windows NT will load a boot device
driver and use that driver to access the boot partition. Then also copy the
drive controller driver to the floppy but renamed to ntbootdd.sys

Something like this below;

[boot loader]
timeout=10
default=scsi(0)disk(0)rdisk(0)partition(1)\winnt
[operating systems]
scsi(0)disk(0)rdisk(0)partition(1)\winnt="Windows 0,1"
scsi(0)disk(0)rdisk(0)partition(2)\winnt="Windows 0,2"
scsi(0)disk(1)rdisk(0)partition(1)\winnt="Windows 1,1"
scsi(0)disk(1)rdisk(0)partition(2)\winnt="Windows 1,2"

[...]

a) It may have been helpful I you had given more infos about your
configuration.
SCSI, IDE, RAID, size of your HDD and system partition

b) Besides a 2nd (parallel) installation of NT4 you can access your NTFS
volumes also by 3rd party software like
    http://www.sysinternals.com/ntw2k/freeware/ntfsdos.shtml (but no write
access)
  or
    http://www.datapol.de/dpd/freeware/index.html (but only German and for
private use only)

This will at least help to read your original boot.ini on HDD if it is
located on an NTFS-partition (otherwise you can read it anyway)

c) If you really did nothing but applying SP6a your troubles might have
something to do with the updated ATAPI.SYS. This applies e.g. for some
DELL-Systems, see
http://forums.us.dell.com/supportforums/board/message?board.id=pes_wnts&message.
id=1985&view=by_date_ascending&page=1
(just an idea)

Stephan