Earliest macOS that will allow adding second backup drive when first backup drive runs out

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Level 1

9 points

Earliest macOS that will allow adding second backup drive when first backup drive runs out

Greetings,

Is it possible on a later version of macOS after 10.13.6 that you can add a secondary backup drive to take over for backups When the first drive runs out of space? If so, could you please let me know what the earliest version is, as I do not necessarily want to go to the latest macOS to minimize any Adobe or Avid apps or plug-insrequiring updated versions also.

I’m currently on macOS 10.13.6 and the backup drive that I’m using for Time Machine is 10 TB but the drive is now full and I need another 4TB of space, I wanted to connect a secondary drive for Time Machine. That is an 8TB drive and use it to back up the additional for 4TB that I need, but on Mac OS 10.13.6, it seems that adding a secondary backup drive only creates a full secondary backup and would require essentially just a larger backup drive to fit everything that needs to be backed up, and doesn’t allow any option to just continue backing up when the first drive runs out of space for back up.

Thank you for any help on this.

Posted on Sep 2, 2025 05:27 AM

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Answer 1

John Galt

Level 10

153,520 points

Sep 2, 2025 10:08 AM in response to djpersyst

The short answer is that every Time Machine backup is a complete, restorable full system backup.

Are you unable to create an additional backup? If Time Machine is showing an error, what is its exact message text?

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Answer 2

djpersyst Author

Level 1

9 points

Sep 2, 2025 11:18 AM in response to John Galt

Thanks for the reply. My Mac OS 10.13.6 just gives me the message of something like “cannot backup, need bigger drive” when I try to “add on” a second backup drive that is 8TB. So it seems that I would need a single 14TB drive to actually get Time Machine to give me any backup.

I thought that maybe one of the new OS versions might allow for me to just add on an additional 8TB drive to pick up where the (already full 10TB backup) original backup drive stopped backing up, and back up the remaining 4TB that could not get backed up in the 10TB drive. Does any newer Mac OS do this type of backup I’m asking about?

thanks

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Answer 3

John Galt

Level 10

153,520 points

Sep 2, 2025 11:43 AM in response to djpersyst

Thanks for the reply. My Mac OS 10.13.6 just gives me the message of something like “cannot backup, need bigger drive”

Thanks but I need the exact message text, verbatim. A screenshot would be most helpful, and might be easier.

The reason for asking is that it would be very unusual for a Mac of any vintage to require a backup drive of greater than 10 TB. What kind of Mac is it, and what is its internal storage capacity? If it is on the order of 5 TB or more then you really do need a larger backup drive. 14 TB would be merely adequate.

Shortcutting your original question, no TM cannot spread its backups over multiple backup drives. They would need to be part of a RAID and I don't recommend that. One drive will contain a bare minimum of one complete restorable system backup.

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