There has to be a setting to only allow a once a day incremental back up.
I don't trust those external drives as the only form of backup, in a business environment. Not only do they overheat easily, and are slow, but it sounds like you could be overwriting on top of the same backup every day. If your database gets corrupted, and then it automatically overwrites your only backup - you might be left with nothing at all.

Incremental can be dangerous, depending on the s/w.
The same concern applies to fire, theft, intentional damage from an employee, virus, or an act of nature. An electrical storm strike would likely blow out the pc AND the connected external drive at the same time.
To back up a multi-pc database, it usually means none of the computers can be USING the program at the time of backup. If someone is using the database, records may be locked - and the important files might be skipped entirely.
For these reasons, I don't suggest leaving a backup running 24/7. I also don't suggest using only one backup device. To be effective, you need multiple generations of backups. (Ie do you want go to back to last night, the night before, or 4 days ago?)
It's probably best to do the backup as part of your daily closing routine, so you can be sure all pc's are out of the application and that none of the files you wish to back up are in use. Backing up our POS only takes 2-5 seconds per day, using flash drives.
I encourage you to check with Carol and Bert for their backup requirements/recommendations, but a series of these may be a very affordable and viable solution:
http://www.buy.com/retail/product.asp?sku=208403525 $9.77 each, shipped. 2gb each
It's also good to have a full backup of your pos folder, documents, accounting, etc. These don't necessarily have to be daily, however. It depends how often those items are usually updated. In our shop, we use a rotation of 6 flash drives to back up LifeSaver every day (we are open 6 days per week, and have them labeled as such).
Every Saturday, I do a FULL backup of the accounting, documents, and pos folders. (I do this via the internet, and store it on the HOME computer).
The first of each month, we burn a 5 cent CDR with the same info, and put those in the fire safe at home.
Are you sure that you need to back up the CMC pc? Very little changes, and if you crash you will need to re-install from the CD and get a new activation code. We copied our folder to an external hard drive, but don't regularly back it up. Any important files/cmc custom jobs are saved to the server machine, and get backed up through the weekly process. We have a shared network drive that the CMC pc can access for this purpose.
I'm sorry this got so long, and hope it has some useful tips.
Mike