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.
I have done this using the raw InstallESD.dmg image from the Mac App Store Lion installer (NOT the Recovery Disk Assistant) using the following steps:
- Drag the InstallESD.dmg image to Disk Utility so that it appears on the side bar
- Select your USB drive and partition the drive in to 2 or more GUID partitions
- Once partitioning is complete, select the Restore tab
- Drag that InstallESD.dmg image from the side bar to the source field
- Drag the first partition from the USB drive on the side bar to the target field
- Begin the restore process
Once finished you can leave things as they are or re-format the other partition as ExFAT or MS-DOS. However you should be aware the USB drive is GUID partitioned so only Windows 7 is likely to read it properly (I've never tried a GUID partioned USB drive on an XP system). You cannot use standard MBR partitions for the Lion Recovery Partition.
Best Answer
They do different things.
Recovery USB allows you to boot your Mac and run recovery software and "investigate" Tue condition of the software, hardware, etc. To use that to reinstall OSX requires an active internet connection (Ethernet I think) to download an image to install again.
Install USB does a'll that, and can perform a full reinstall with no internet connection.