Q: What happens if external source isn't there?
I'm backing up my MacBook Pro and an external media drive to my Time Capsule. When I travel, I never take my media drive with me. What does Time Machine do when it can't locate either the Time Capsule or the source?
Posted on Mar 16, 2016 5:05 PM
If you have a Mac laptop, Time Machine makes "local snapshot" backups to the laptop's internal drive, then resumes normal backups when it knows that other drives are attached. Later, the backups will appear "merged" when you open Time Machine normally to browse backups.
You might want to look though this Apple support document for more details on "local snapshots":
About Time Machine local snapshots - Apple Support
Use Time Machine to back up or restore your Mac - Apple Support
Posted on Mar 16, 2016 5:19 PM