Can't boot from thumb disk

[Kim and Alan]

All,

I have used the 5.2 released version of the iso disk 1 to install the 
"minimal" installation onto a 256M "thumb disk".

i know the installation completed and I can mount the disk on another Freebsd 
machine and see that all the files seem fine.

If I don't install the boot selector then the bios reports "no operating 
system found"

If I do install the boot selector then I get the boot0 prompt "F1 FreeBSD, 
Default F1" (So the Bios can boot from USB...)

But then I get a short beep - the LED on the Thumb drive flashes and I get no 
further prompts. This happens again when I hit any key.

I see other questions in the archives about this but no answers (at least that 
I can understand). 

Please help