Mac HD - Data, MacOS Catalina, and Time Machine

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Mac HD - Data, MacOS Catalina, and Time Machine

Is there an issue with the newer Mac Catalina separation of system and user data that causes the bulk of your unused HD to become invisible to the OS, resulting in repeated prompts saying that your Mac HD disk is full, when you should have 800-900 GB available? I have read other people’s frustration with this and the one thing I can find that I have in common with them is a potential issue with Time Machine.

After connecting a new external Samsung 500GB SSD drive I was prompted to use it for Time Machine. After confirming this it attempted to make its first backup of something less than 1TB, but (pretty sure it was, though I don’t recall how much) greater than 500GB and so failed to complete the backup. I’m from the old school where we would drag and drop data onto SCSI externals for backups so not being able to simply dump the incomplete back up that is making my Mac HD think there is no space left has me scratching my disc/head.

Given that losing existing data will not cause any serious issues would reformatting the Mac HD in Recovery mode for a clean Re-Install be my best/safest option? Should I reformat the external SSD also before trying to use is for Time Machine?

iMac 27″, macOS 10.15

Posted on Jun 17, 2021 4:19 AM

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Answer 1

Best reply

John Galt

Level 10

142,150 points

Jun 17, 2021 6:25 AM in response to transcendentalaccidentalism

Don't reinstall macOS because that won't help in your case. You need a bigger TM backup device. 500 GB is a paltry amount these days.

As of Catalina, Time Machine no longer backs up an autonomous copy of macOS itself. TM creates a bootable Recovery partition, but the only thing you can do with it is boot Recovery so that you can reinstall macOS.

Forget the old school way of doing things. You can still create archival copies of data in the manner you describe, but TM is much, much more than that. From a completely technically accurate perspective, the required minimum capacity of the backup disk is a function of the amount of data that needs to be backed up. As such, the capacity of the source volume(s) isn't a factor. However, the amount of data that needs to be backed up is not easily calculated. Then, bear in mind TM always retains a minimum of one completely restorable backup even before it begins to create the next incremental backup. Only after it finishes that new backup will it begin to erase older, "expired" backups. When determining how much space it needs, TM always overestimates it. That way, you are never without at least one complete backup.

The bottom line is that if TM says it needs a larger backup disk, then that's what you need.

I wouldn't bother using an SSD for TM. Speed isn't important. Redundancy is.

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Answer 2

tbirdvet

Level 6

13,845 points

Jun 17, 2021 5:05 AM in response to transcendentalaccidentalism

I have found that TM wants to be at least the size of the internal drive (or larger) or it may state "not enough room" regardless what the actual data size is. That is the way TM appears to want to operate. Can you get another drive (perhaps inexpensive mechanical) that is larger and see if TM will function correctly? I would not reformat your internal drive at this point. You will need to format the new mechanical drive of course.

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Mac HD - Data, MacOS Catalina, and Time Machine

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