[RKDinOKC]

I keep my boot drive space small, 1 GB and use a 1 GB external drive to do a bootable backup of the boot drive. Also use a server to do Time Machine backups, and backup the Time Machine backup. Time machine requires at least 3 times the drive space of the original because it backs up versions of the files you are working on. Don't use time machine much, but it does save versions of files.

That's how I use my smaller external storage devices. Has saved me when a logic board died. Just booted my extra machine off the bootable backup and was back in business then got the broke machine fixed/updated.