Restoring data from Carbonite and an external drive

I have been a happy user of Carbonite's cloud backup for a couple of years. I'm up to about 220 GB of data backed up. During this time, I've attempted two restores of my data, both successfully.

The first restore was due to a hard drive failure. Upon rebuild of the PC, I restored everything without issue. The second restore I attempted recently when I moved to a new desktop PC.

Carbonite and External Hard Drive Hybrid backup.

If not Carbonite, I strongly urge everybody to have some sort of off-site backup. However, just as relying on one's hard drive to not fail is flirting with disaster, I also think it's sub-optimal to rely on a single cloud-based solution. As such, my preference is to occasionally back everything up to an external drive (if not two on a rotating schedule--it may sound unnecessary but mechanical hard drives all have life spans).

Carbonite has documentation on restoring, but I couldn't find much online about restoring after a catastrophe first from an external drive, and then using Carbonite to fill in any recent gaps. This is preferable because if the bulk of data can be restored from a local source total restore time will be much quicker than downloading everything over the net. This is a similar approach to Carbonite's courier recovery (they mail you a data dump if you need to restore) minus the additional costs of that approach. So, I tried it this way. The following are the steps I took:

1. Copy all data from the external drive to the PC drive. Folder structure and files were not changed whatsoever, and went to the same location on c:\ they originally had been
2. Installed the Carbonite client and, of the various setup options, chose the one about moving Carbonite to a new computer
3. Carbonite went through its restore steps. First it builds folder structure, then it restores files. But wait, weren't the folders and files there already? Yes, and evidently Carbonite is smart enough not to recreate them.
4. 2-3 hours later the process was complete. Restore report popped up, and Carbonite then offered me the option to unfreeze my backup, and begin backing up as normal

My internet speed can't move 220 GB in <3 hours, so clearly Carbonite functioned as hoped: it's smart enough during a restore to not overwrite already-existing files. In retrospect this would have been a good chance to test its gap-filling ability: I should have deleted a few local files before the restore. I didn't do that, but it seems entirely reasonable to think it would have restored missing files.