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Small SCSI drive not working with DOS FDISK

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First post, by dionb

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Having some SCSI headaches here... I have a Miniscribe 8425S SCSI drive I want to use as boot disk.At 425MB it's by no means large, either for BIOS or for FAT16. Still, both DOS 6.22 and DOS 7.1 FDISK identify it as a 20MB drive. Regardless, I can format it to the full ~400MB net space and use it in DOS - but even though the single partition on it is set to active, I can't boot off it.

It's definitely not a size issue vs BIOS or DOS, as the same combination of motherboard and DOS 6.22 or 7.1 happily accept a Seagate ST3660A 660MB IDE HDD, detect its full size in FDISK and boot from it.

The SCSI drive is connected via an Adaptec AHA-1542C, which shows up during boot, initializes its BIOS and proclaims the 8425S as drive C:. For troubleshooting purposes I've removed all other I/O controllers in the system, it's just the MG TK8498F motherboard, a Miro Crystal 24S (S3 928 3MB VLB VGA) and this AHA-1542C. Only other drive is a Gotek on its floppy controller, which boots happily.

I'm pretty sure the problem is with incorrect partitioning, but can't figure out why it should be such a problem. Any tips?

Reply 1 of 6, by yawetaG

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Did you try disabling the IDE controller in the BIOS?

Besides that, IIRC, the SCSI ID on the drive must be set to a particular ID to be able to boot off it (I think "0"). You also may want to play with some of the settings in the AHA-1542C SCSI BIOS, some drives need a pause before the controller tries to boot off them.

Reply 2 of 6, by PC Hoarder Patrol

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Sounds a bit like the old alignment problems you used to get when swapping controller brands / chipsets. Have you considered a low-level format thru the Adaptec boards SCSI Disk Utilities menu?

Reply 5 of 6, by dionb

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yawetaG wrote:

Did you try disabling the IDE controller in the BIOS?

This 486 board doesn't have onboard IDE, I physically removed the controller 😉

Besides that, IIRC, the SCSI ID on the drive must be set to a particular ID to be able to boot off it (I think "0"). You also may want to play with some of the settings in the AHA-1542C SCSI BIOS, some drives need a pause before the controller tries to boot off them.

The drive is set tot ID 0, is recognised by SCSI BIOS and it sets it to C:

PC Hoarder Patrol wrote:

Sounds a bit like the old alignment problems you used to get when swapping controller brands / chipsets. Have you considered a low-level format thru the Adaptec boards SCSI Disk Utilities menu?

Hmm, worth a try... will do so this evening.

SW-SSG wrote:

Are you sure you have the correct model number? The 8425S is a 20MB drive. There's a stepper motor on there instead of a voice coil actuator, right?

Hang on, yes I am sure the model number is correct, and there is definitely a stepper - you can see it turn outside of the platter enclosure, complete with warning sticker not to touch it.

So it's a 20MB drive. Fdisk isn't mad. I am. Or rather, whatever lead me to believe it is 425MB is either misleading or just misread 😳

Reply 6 of 6, by dionb

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Right, it was the motherboard (BIOS?) after all.

Tried the low-level format. Didn't change a thing.
Moved it to another 486 board: immediately booted from it.

Then moved back and started combing through the BIOS: despite not having onboard I/O there were settings related to disabling secondary master/slave, but nothing to disable primary master. So it looks like BIOS is hard-coded into expecting an IDE primary master when it tries to boot from C:

Given that it is also clearly a 20MB drive, not a 425MB one, I need to reconsider my intentions, as 20MB is a bit on the low side for a ~1993 EISA/SCSI build...

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