Comment # 1 on bug 1000947 from Michal Suchanek

The reason this fails is probably that whole medium is checked and you do not
know the length of the medium when copied to an USB stick which will typically
have more space. This is not unsolvable problem, however. The iso9660
filesystem has length information somewhere for sure. It might be difficult to
account for rounding, though. A simple alternative is to save both checksum and
length. then there is no reason for check to ever fail.

Ubuntu medium check checks only individual files and so can work on any medium.
It cannot verify integrity of bootloader this way, though.