How do I fix a "The disk was not ejected properly" error?

macOS only. If you see this error, it means one of a few possible things:

1. That you're trying to scan a document while Doxie is importing scans. Wait until your Mac is done importing before trying to scan new items.
2. That you've unplugged your scanner while Doxie's memory is in use.
3. That Doxie's memory and your Mac have not yet been enabled for connected scanning. (How do I enable connected scanning?)