I'm just going to take a stab in the dark here and say that Syslinux probably isn't able to locate ldlinux.sys. ldlinux.sys is added by Syslinux when it the bootloader is installed. It is the second stage of the Syslinux bootloader (the whole bootloader doesn't fit in 412(?) bytes). The deal is, though, that it has to be at a specific point on the disk. If it's been moved from that specific location (perhaps via copy/paste) then Syslinux will not be able to find it.
The key to fixing this is to find syslinux.exe (I'm assuming you're using Windows for this process) on your drive. If you don't know where it is, look in unetbooting/pendrivelinux Program Files folders.
- Take note of the above location
- Open a command prompt (as Administrator)
- BE VERY CAREFUL HERE THAT YOU SPECIFY THE CORRECT DRIVE LETTER!
syslinux -m -a F: (where F: is the drive letter of your USB drive.)
- syslinux will then re-add the ldlinux.sys file, but in the correct location so that it can be read at boot.
Hopefully that should rectify your booting situation.