isolinux.bin missing or corrupt, what is the next step in debugging?

I am trying to boot a Ubuntu 14.04 live system on a machine that is currently unable to boot from its harddisk due to the installed OS having gotten corrupted somehow.

I used this command to write the install image to the USB device

# cat /var/tmp/ubuntu-14.04.1-desktop-i386.iso >/dev/sdb

When trying to boot, I got an error message saying isolinux.bin missing or corrupt. Nothing else was output to the screen during boot.

Afterwards I verified the integrity of the USB media again like this:

# head -c1034944512 /dev/sdb | sha256sum 
5cd41b34c86799c92d316718d9763d225847bbc290aba0dc312623418e02d6cf  -

This appears to be the correct checksum, so I have no reason to think the image is corrupted. What else could be the reason for this error message?