Time Machine provides easy access to mounting a backup of a volume that’s not currently plugged in.
You can keep an ongoing deep backup of not just your startup volume with Time Machine but any qualifying drive you connect to your Mac. The provisos? It has to be formatted as HFS+ or APFS, and cannot be a Time Machine volume.
But what happens if you don’t always have one of your external drives plugged in—how do you access its Time Machine backups if you can’t use the Time Machine system menu’s Enter Time Machine option with the volume open in a Finder window? Easy: use the backups directly on your Time Machine volume.
Read more at MacWorld.com