It sounds like the USB drive doesn't have the drivers to boot the other macs. :-(

The fastest path forward is to simply erase install a basic Snow Leopard OS onto the failed macs.

Don't bother with updates unless the installer forces you to get to a higher version than your 10.6 installer level.

Once you boot into OS X - you can still mount the USB and try clicking the package to execute the Lion upgrade program. I don't know if it runs mounted or needs to be copied to the internal boot drive.

There's a little uncertainty in my brain - so I don't want to write too much without making sure this makes sense to you.

It sounds like the USB drive doesn't have the drivers to boot the other macs. :-(

The fastest path forward is to simply erase install a basic Snow Leopard OS onto the failed macs. While this is happening, do download the 10.6.8 Combo update to your USB drive just in case it's needed.

Don't bother running the updates unless the Lion installer forces you to get to a higher version than your 10.6 installer delivered.

Once you boot into Snow Leopard - you can try again to mount the USB and execute the Lion upgrade package. I don't know if it runs well from the USB or needs to be copied to the internal boot drive.

There's a little uncertainty in my brain - so I don't want to write too much without making sure this makes sense to you.

As an alternative - you could try instead to transfer the recovery partition, but this may not be universal (include the drivers the older macs) either.

There is a step-by-step recipe here for copying any bootable volume to one file on a USB drive.

If you are curious or feel it's worth a shot, image the recovery partition from your Lion mac.

You should be able to boot from DVD and use disk utility to make an equivalent partition on the "non bootable" mac and drop the recovery data to get a minimal bootable system and avoid a full Snow Leopard install.

Unless you are familiar with Disk Utility and the steps to capture, the reinstall option might be more likely to succeed on first attempt. I certainly don't know if this partition is customized by Lion and not universal so I've made it an aside for the curious.

It sounds like the USB drive doesn't have the drivers to boot the other macs.

The fastest path forward is to simple erase install a basic Snow Leopard and boot the mac into Snow Leopard.

Don't bother with updates unless the installer forces you to get to a higher version than your 10.6 installer level.

Mount the USB and try clicking the package to execute the Lion upgrade program.

There's a little uncertainty in my brain - so I don't want to write too much without making sure this makes sense to you.

It sounds like the USB drive doesn't have the drivers to boot the other macs. :-(

The fastest path forward is to simply erase install a basic Snow Leopard OS onto the failed macs.

Don't bother with updates unless the installer forces you to get to a higher version than your 10.6 installer level.

Once you boot into OS X - you can still mount the USB and try clicking the package to execute the Lion upgrade program. I don't know if it runs mounted or needs to be copied to the internal boot drive.

There's a little uncertainty in my brain - so I don't want to write too much without making sure this makes sense to you.