17 Nov
2025
17 Nov
'25
1:33 p.m.
I have an ST-251 42MB MFM hard disk drive.
The documentation says the drive is 512 bytes per sector. I don't know
if this is fixed (hard sectored) or not.
My problem is my operating system (OS/9 Level II) wants 256 bytes per
sector. The OMTI 20C-1 controller can handle that but I'm not sure if
the ST-251 can.
Does anyone know if the ST-251 can support 256 bytes per sector and/or
how to configure it for 256 bytes per sector?
Thank you,
Mike
17 Nov
17 Nov
1:48 p.m.
Does anyone know if the ST-251 can support 256 bytes
per sector
and/or how to configure it for 256 bytes per sector?
I'm pretty sure that on an MFM hd, this is determined by the low level
format that's applied to the drive. The LLF is basically determined by
the controller.
De
2:10 p.m.
On Mon, Nov 17, 2025 at 4:41 PM Mike Katz via cctalk
<cctalk(a)classiccmp.org> wrote:
I have an ST-251 42MB MFM hard disk drive.
The documentation says the drive is 512 bytes per sector. I don't know
if this is fixed (hard sectored) or not.
It is not.
My problem is my operating system (OS/9 Level II)
wants 256 bytes per
sector. The OMTI 20C-1 controller can handle that but I'm not sure if
the ST-251 can.
It can.
Does anyone know if the ST-251 can support 256 bytes
per sector and/or
how to configure it for 256 bytes per sector?
You don't configure MFM drives for that. The LLF on the controller
takes care of it.
You can stuff an ST-251 into a Commodore D9090 hard drive box and it
will format to 32 sectors of 256 bytes per sector. Works great.
-ethan
2:31 p.m.
Thank you both for answering.
On 11/17/2025 4:10 PM, Ethan Dicks via cctalk wrote:
On Mon, Nov 17, 2025 at 4:41 PM Mike Katz via cctalk
<cctalk(a)classiccmp.org> wrote:
I have an ST-251 42MB MFM hard disk drive.
The documentation says the drive is 512 bytes per sector. I don't know
if this is fixed (hard sectored) or not.
It is not.
My problem is my operating system (OS/9 Level II)
wants 256 bytes per
sector. The OMTI 20C-1 controller can handle that but I'm not sure if
the ST-251 can.
It can.
Does anyone know if the ST-251 can support 256
bytes per sector and/or
how to configure it for 256 bytes per sector?
You don't configure MFM drives
for that. The LLF on the controller
takes care of it.
You can stuff an ST-251 into a Commodore D9090 hard drive box and it
will format to 32 sectors of 256 bytes per sector. Works great.
-ethan 
6 p.m.
The controller is the entire hardware involved in the "low level" format.
'course you also need appropriate software to do it.
In the days of the 5160 (XT) Low level format software was not bundled
with them, nor with the OS. The only source for a while was that there
was copy of the IBM low level format program on the "ADVANCED Diagnostics
disk.
Fortunately, third parties jumped in and you had commercial programs for
the purpose, such as SpeedStor. Most controllers had the low level format
code in ROM, but they didn't OFFER it to you. You could however go into
DEBUG and jmp to C800h: 000x (which varied a little between controllers).
If you've got a compatible controller, and the right software for that
controller, then you are all set.
--
Grumpy Ol' Fred cisin(a)xenosoft.com
2:13 p.m.
On 11/17/25 22:33, Mike Katz via cctalk wrote:
I have an ST-251 42MB MFM hard disk drive.
The documentation says the drive is 512 bytes per sector. I don't know if this is
fixed (hard sectored) or not.
My problem is my operating system (OS/9 Level II) wants 256 bytes per sector. The OMTI
20C-1 controller can handle that but I'm not sure if the ST-251 can.
Does anyone know if the ST-251 can support 256 bytes per sector and/or how to configure
it for 256 bytes per sector?
Thank you,
Mike
I am using an ST251 with 256 bytes/sector in an ETH Lilith.
The WD1001 controller takes care of everything.
Jos
5:47 p.m.
On 11/17/25 15:33, Mike Katz via cctalk wrote:
I have an ST-251 42MB MFM hard disk drive.
The documentation says the drive is 512 bytes per sector.
I don't know if this is fixed (hard sectored) or not.
My problem is my operating system (OS/9 Level II) wants
256 bytes per sector. The OMTI 20C-1 controller can
handle that but I'm not sure if the ST-251 can.
Does anyone know if the ST-251 can support 256 bytes per
sector and/or how to configure it for 256 bytes per sector?
MFM drives are formatted entirely by the controller. The
one thing the drive requires is the correct bit clock rate.
There is a write clock and a write enable signal on the
20-pin connector.
Jon
8:57 p.m.
On Mon, Nov 17, 2025 at 2:41 PM Mike Katz via cctalk <cctalk(a)classiccmp.org>
wrote:
I have an ST-251 42MB MFM hard disk drive.
The documentation says the drive is 512 bytes per sector. I don't know
if this is fixed (hard sectored) or not.
My problem is my operating system (OS/9 Level II) wants 256 bytes per
sector. The OMTI 20C-1 controller can handle that but I'm not sure if
the ST-251 can.
Does anyone know if the ST-251 can support 256 bytes per sector and/or
how to configure it for 256 bytes per sector?
I have (well had) a ST-251-1 (so basically the same drive) that had 2
tracks formatted with 256 byte sectors, 2 tracks with 128 byte sectors and
the rest 512 byte sectors. The first 4 tracks were for some funky
'partitioning' software that either the controller or the disk driver for
the OS needed. It was some weird Epsom system, I think. It's been a while
and I traded it away years ago.
Warner
18 Nov
18 Nov
1:54 p.m.
I am pretty sure it is soft sectored so that is strictly a controller function
see: APPENDIX 1: 32-Sector, 256-ByteiSector Format Example ............ 25
http://www.bitsavers.org/pdf/seagate/mfm/ST-251/36016-001A_ST251_OEM_Manual…
tom
-----Original Message-----
From: Mike Katz <bitwiz(a)12bitsbest.com>
Sent: Monday, November 17, 2025 1:33 PM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Subject: [cctalk] ST-251 MFM Hard Drive sector size question
I have an ST-251 42MB MFM hard disk drive.
The documentation says the drive is 512 bytes per sector. I don't know if this is
fixed (hard sectored) or not.
My problem is my operating system (OS/9 Level II) wants 256 bytes per sector. The OMTI
20C-1 controller can handle that but I'm not sure if the ST-251 can.
Does anyone know if the ST-251 can support 256 bytes per sector and/or how to configure it
for 256 bytes per sector?
Thank you,
Mike
1
days inactive
2
days old
8 comments
8 participants
participants (8)
-
Dennis Boone -
Ethan Dicks -
Fred Cisin -
Jon Elson -
jos -
Mike Katz -
Tom Gardner -
Warner Losh