I already have my MacBook backing up to a NAS via Time Machine, but I want a sort of belt-and-suspenders option, too. Specifically, I have a small (140 MB) folder with important documents that are periodically modified (i.e., worked on) that I would like to have backed up to a second volume, in this case a 16 GB flash drive. I'm looking for an app that will first back up the folder, then back up those incremental changes--i.e., like Time Machine, not replacing the old versions, just adding on new ones. It would help if the app did so automatically whenever the volume is mounted (that is, when the flash drive plugged in), then unmounting the flash drive. Version management, of course, would be very helpful. It looks that SuperDuper (which I've used for more generic backups) can't do this, as it only backs up the whole volume, or the User files.