Time Machine backups for internal and external drives

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Level 1

10 points

Time Machine backups for internal and external drives

After a drive failure, I had to redo a fresh Time Machine back up. I backed up my internal drive and 6 external drives (four in an enclosure.) It took almost 10 hours. Fine no problem. Now for the following back ups, do I have to have the same external drives connected, or can I just backup the internal drive, leaving the external off, until they need to be copied as well?

[Re-Titled by Moderator]

Original Title: Time Machine sources

Mac Studio

Posted on Aug 22, 2025 06:50 PM

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Answer 1

HWTech

Level 9

66,674 points

Aug 22, 2025 07:23 PM in response to Guerin

AFAIK, TM will backup any drives which are connected at the time a backup is initiated. If a drive is not connected, then it will be ignored until it is reconnected. I don't know how soon TM will initiate a backup once a drive is connected.

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Answer 2

Guerin Author

Level 1

10 points

Aug 23, 2025 05:55 AM in response to HWTech

Thank you. What I meant more precisely: will Time Machine use the first back up, including the back up of all the external drives as a base, or will it backup anew all the connected drives, because the latest back up did not not include them, resulting in a 10 hours+ back up ? Not what I want.

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Answer 3

Aug 28, 2025 12:27 AM in response to Guerin

Time Machine keeps track of each source drive individually. If you disconnect the externals, TM just skips them and keeps backing up the internal.

When you reconnect those externals, TM doesn’t start from zero, it looks at what’s already backed up and only copies new or changed files.

The only time you’d get another 10-hour marathon is if you wiped the backup or reformatted those externals, because then TM would see them as new drives.

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Answer 4

Aug 28, 2025 06:18 AM in response to HWTech

TM will backup any drives which are connected at the time a backup is initiated.

This is not correct. When you connect an external drive to a Mac, it will not be automatically backed up by Time Machine. The drive will appear in a list of drives that are NOT being backed up by Time Machine.

If you want the drive to be backed up by Time Machine you will need to go into Time Machine options to remove the drive from the list of drives that are not being backed up. Then.....the drive will be backed up along with the Mac on the next Time Machine run.

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