Tried that and still getting the same error. Syslinux seems to be completely un-interested in the memory key.

fdisk reports the key is partitioned as follows:

Device Boot Start End Blocks Id System
/dev/sda4 * 1 1023 4190144 6 FAT16

I'd like to get this working but it looks like I might have to give up and go get a 1GB key which has been proven to work.