Comment 70 for bug 658865

Got sick and tired of this bug so figured I'd find out what the cause is. Turns out (for me) because I overwrote multiple versions of Linux using unetbootin, there were multiple apt folders in dist/ on the drive (i.e. yakkety, zesty, etc) so when the 40cdrom script came around, it tried to validate the GPG signature of all the dist/ repos including the non-target dist/ which of course fails so it bubbles up as a failure of the whole installation script. Removing the folder in question made the installation proceed without issues.

PS: Without 40cdrom script, latter part of the installation fails if you have a UEFI system since it won't be able to find the signed grub shim so this approach is not recommended.

tl;dr fix: Removing the folder not matching installation CD version from your USB dist/ folder should make it work.