I apologize for the delay in response.
How I made my flash.
When I worked on my project of bootin'g usb-flash - I found a simple way to make it bootable:
Connect flsh to your computer and boot MS-DOS from Windows 98. flash-drive will be assigned some letter. for example "F:"
Then run sys F: and our flsh-drive is bootable with MS-DOS!
Furthermore I'm use GRUB4DOS for booting other things like linux, acronis etc.
On modern computers that support booting from UBS - everything works fine!
But I install plop boot manager on the computer where the BIOS does not support booting from USB and received a message "Disk error".
You tell that "the error message is not programmed into the boot manager".
In this case, I think the message comes from the BIOS. (

)
However, instead flash-drive I try to boot from IDE HDD connected via USB-IDE adapter.
On that HDD installed windows xp. But again there is the same error message.