Hi all,

I went around in circles on this one so I'm posting here for posterity in case this info helps someone else on day.
I've just done an EISA build using the BusTek BT-742A SCSI controller. This has support for >1GB disks, but unfortunately it was not working.

With >1GB support enabled, fdisk could see the whole disk and create a large partition... however the machine would crash when trying to boot again from floppy to format the drive. (The controller also wouldn't boot from an existing system formatted drive - "Operating system not found"). If trying to boot with a DOS version that supported the partition, the machine would crash upon checking for partitions. Eg if it's a FAT32 partition then a DOS 6 boot disk would work but not Win95. With a FAT16 partition the Win95 boot would crash too. Similarly if there was a >1GB partition on a secondary drive (sidenote: drive C must be SCSI ID 0, and drive D must be SCSI ID 1, otherwise fdisk will not see the second fixed disk).

Here's my card:

I finally gave up and decided to stay with a 1GB drive... and then had issues with getting a SCSI CDROM drive to work. BTDOSM.SYS gave an error about being unable to configure the card.
So I went back to Googling for drivers... and this time I finally stumbled upon a couple of important files. 👍 😄

In short, it just needed a BIOS and firmware update (4.7 and 3.37 respectively). My card must have had buggy versions. After burning a couple of W27E257s there were proper messages about >1GB support. There was, however, still an issue with it reporting that the existing disk partition didn't match the setting, and fdisk was limited to 1024MB. Enter info #2... I found an installation guide (text file) that mentioned needing to low-level format drives using debug. The critical line was missing but easy enough to find with Google: g=dc00:6 (or whatever you've set the base address to in the EISA config utility).
BTDOSM.SYS and BTCDROM.SYS then also worked properly (these are easy to find, and are on the Win98 boot disk).

I've attached the files as found online.

This page was also helpful: https://www.os2museum.com/wp/a-brief-visit-to … -geometry-hell/

Hope this helps save someone else some time and headaches...