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.

The Basics: Time Machine

Time Machine is Apple's backup solution, included in OS X. An external drive is required. It backs up changed files every hour, and stores as many "snapshots" as it can hold. After the initial full backup, it only stores changes to files, which makes things more efficient, and allows for more granular snapshots. In addition to being able to do a full system restore, you can selectively restore older versions of specific files, which is handy in case you delete or make changes you didn't mean to.

Using Time Machine on OS X 10.7 and later also enables "local snapshots", which keeps snapshots of changed files on your main drive when you don't have your backup drive attached, provided there's sufficient space. Obviously this doesn't help if you have a drive failure, but in the case of accidental deletions, etc. it can be helpful.

For more information, Apple's Mac 101: Time Machine article is a good starting point.

Online Backup

If you're on the go a lot, you may want to consider an online backup service. BackBlaze is a popular one, but there's also CrashPlan, Mozy and several others.

The advantages are not needing an external drive, and having an off-site backup to recover from in the event of something nasty happening to your house. The downside is the monthly cost (I believe BackBlaze is a flat fee, others have differing schemes based on how much data you back up), as well as the time required to back up over the internet. These services only back up data that has changed, so after the initial full backup, things will get faster, but depending on your connection, it could be a bit problematic (particularly if you're travelling — most hotel Wi-Fi I've used isn't exactly speedy).

SuperDuper

SuperDuper is a longstanding Mac favourite. It will make a complete clone of your drive to an external drive, which is then bootable and usable exactly like your internal drive. It also has a "smart update" feature, where after the initial clone, it only updates the backup with changed data, to minimize how long backups take. SuperDuper is the best way to get up and running again after a hard drive crash — you can boot off your backup drive right away and you'll your full system.

Dropbox

I'm listing this separately from the other online backup systems because it's not really a full backup, but it's definitely worth using. It's simply a folder that syncs to the Dropbox servers and wherever else you install your Dropbox client. Some limited storage of old versions of files is also included. 2GB is provided free, you can pay if you need more. The best use of Dropbox would be for your current working files. It won't store everything, but if you use it for the files you most often need, or those that you're using lately, you'll have up to date backups from whenever you have an internet connection.

Wrap Up

You'll have to do think about what exact setup suits your needs best. Ideally, a robust backup system would include at least two or three of these options in combination, so that you have multiple fallbacks. For example, you could do a daily or weekly SuperDuper backup, and then use Dropbox or Time Machine to restore the files you've been working on since your most recent backup. Adding in an online backup service for off-site redundancy will give you some added protection in the event your local backups are lost or destroyed.