mboot.c32: not a COM32R image
Having followed the instructions exactly from here I couldn’t understand where i was going wrong. I had downloaded the latest copy of syslinux (4.04) and I was receiving the above error despite what boot option i tried. I then noticed my error lower in the comments suggesting I try syslinux version 3.86, which resulted in a flat out “boot error” before even loading the xenserver boot menu.
This may seem like a no brainer to the strong linux guys, but to the rest of us this was quite an annoying little problem. After much head scratching and troubleshooting, the simple and easy solution was to browse into my latest copy of downloaded syslinux > com32 > mboot
copy the mboot.c32 file, then paste it onto the root of the memory key, replacing the existing file:
Once you’ve done the above, reboot the host and try booting from the key again. It should be plain sailing from here.