Comment 9 for bug 1575652

Ah I found another workaround / hack someone left on a comment in my original askubuntu question.

It's definitely a hack but it got me around this bug. I extracted the initrd.gz file served by my TFTP site using this guide: http://www.thegeekstuff.com/2009/07/how-to-view-modify-and-recreate-initrd-img/

Then, I edited the included bin/fetch-url file to add the following lines before the proto= line near the bottom:

# Remove /cdrom prefix:
prefix="/cdrom"
url=${url#$prefix}

Then I repackaged the initrd.gz file, copied it back to my tftp server and everything worked.

Definitely a nasty hack, but it got the image working for me.