Hello again!
I just wanted to report back for all of you who helped me and gave great 

Thanks for the CHS/LBA info.
This is an IDE Flash module that goes directly into the MBs slot. It 
doesn't allow to connect another device beside it, so it also doesn't 
have a master/slave jumper.

It has a capacity of 512 MB. I tested it on another PC, from around 
2000, which auto-detected it as C/H/S 993/16/63 (no boot-up as it was 
still empty at that time, I just did the BIOS auto-detection).

On the 486 I've put that in for the IDE device as custom type 82. The 
other values, LZ and WP, I left at 0. I assume for a flash drive they 
are simply not important.

But it doesn't boot. It halts with the error message: "partition 
signature != 55AA"

I've personally checked: all partitions and the MBR have that signature.

I managed to find an old external USB HDD which has an IDE interface 
inside, and I connected the flash module to my regular PC.

 > # dmesg | tail
 > [23705.522176] scsi 7:0:0:0: Direct-Access     TRANSCEN 
D                0000 PQ: 0 ANSI: 0
 > [23705.522746] sd 7:0:0:0: Attached scsi generic sg3 type 0
 > [23705.523265] sd 7:0:0:0: [sdc] 1000944 512-byte logical blocks: 
(512 MB/489 MiB)
 > [23705.524141] sd 7:0:0:0: [sdc] Write Protect is off
 > [23705.524145] sd 7:0:0:0: [sdc] Mode Sense: 27 00 00 00
 > [23705.525012] sd 7:0:0:0: [sdc] No Caching mode page found
 > [23705.525018] sd 7:0:0:0: [sdc] Assuming drive cache: write through
 > [23705.528556]  sdc: sdc1 sdc2 sdc3
 > [23705.531653] sd 7:0:0:0: [sdc] Attached SCSI disk

 > # parted /dev/sdc print
 > Model: TRANSCEN D (scsi)
 > Disk /dev/sdc: 512MB
 > Sector size (logical/physical): 512B/512B
 > Partition Table: msdos
 > Disk Flags:
 > Number  Start   End     Size    Type     File system  Flags
 >  1      32,3kB  52,7MB  52,7MB  primary
 >  2      52,7MB  74,0MB  21,3MB  primary
 >  3      74,0MB  480MB   406MB   primary  fat16        boot

As you can see, the third primary partition is for FreeDOS. I've decided 
to create a 50 MB partition for DR DOS and a 20 MB partition for MS-DOS 
or any other DOS I want to test (like PC DOS, PTS-DOS).

The third partition has been made bootable with "format c: /s /q". I've 
also rewritten the MBR using "fdisk /mbr". All partitions are formated. 
The first two partitions are otherwise empty.

I've also used Linux's dd (e.g. "dd if=/dev/sdc1 bs=512 count=1 
of=bootsec1.bin") to check if the boot signature 55AA is present on all 
partitions + on the MBR and it is.

Thinking logically: If the BIOS wouldn't check, it should boot. The MBR 
code should find the active partition #3 and chainload the bootsector. 
This should include the bootcode for FreeDOS, since I've formated it 
with /s and the system files are on the parition.
I hate it when a BIOS checks stuff that it rather shouldn't care about!

Any other error message and it could have been FreeDOS, but I am sure 
this one if from the BIOS _before_ it executes the MBR boot code.

Maybe the next time I will try only one partition first... Anyway: 
again, I've come to a halt for now.

I appreciated your help.
Happy 2017.

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