I can enter Time Machine even though the backup disk is unplugged! I have been using for some time Time Machine to backup to an external hard drive. I have just noticed todqy that even when my external hard drive is unplugged I can enter the Time Machine and go a few days back.

How is it possible? Is it a new feature introduced by Sierra?
No - local backups were introduced with Lion. You can enable and disable them manually and they are turned on when MacOS detects a hard drive and portable Mac are separated or likely to run separately. The backups clear when you run low on space so most people never need to manage them. 


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