Grub on USB Key works on desktop but cannot find menu.lst on laptop

[Rich Stanton]

I'm trying to boot from a USB key, I've tried using grub4dos to install
grub1 or grub2, and I've also used syslinux to boot, then hand over control
to grub.  In all 3 cases the USB key boots perfectly on my desktop machine,
but when I try and use it on my laptop it boots ok, but then fails to find
the menu.lst on the usb key.  Instead it runs through the hard disk
partitions.  Using the syslinux method, I end up at a grub menu but instead
of having the entries from the usb keys menu.lst, the options are all simply
searches for menu.lst.  Menu.lst is in the root of the usb key, however
running the search options doesn't result in it finding menu.lst.  I can get
to the grub command line.  If I do 'find' after booting the USB key on the
desktop, it's clear that the desktop is calling the USB key hd0 whereas the
laptop is calling if fd0.  After booting the USB key on the laptop & getting
to the grub command line, if I do root (fd0) then grub correctly says it's a
FAT partition and will use the whole disk.  But running find /menu.lst
returns file not found.

Any ideas what I'm doing wrong - why does the laptop boot the USB key ok,
but then is unable to find menu.lst, whereas the desktop finds it fine?

Thanks for any advice.