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A family member has updated their MacBook Air from the latest version of Mojave to Catalina 10.15.1.

The MBA has got a 250GB SSD drive, of which only 80GB are in use (170 available). They've been backing up the MBA with Time Machine more or less regularly, with the last backup completed 7 days before the upgrade. (I know, I know: they should have backed up right before upgrading, but now it's too late for that) The Time Machine drive is an external hard drive connected via thunderbolt (v1) cable and thunderbolt to USB-C adapter.

Now the upgrade to Catalina has completed successfully, but they can't backup to Time Machine anymore because "there isn't enough space".

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The Time Machine drive is a 1TB hard drive with 505GB available. That should be plenty to perform a new incremental backup of the MBA. I know that backups following a major macOS update take more space, but it should still fit comfortably in 505GB, and even if it didn't it should automatically clean old backups.

Is it a genuine issue (it actually needs more than 500GB), or an error? How do I fix it without losing the old backups?

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I had a similar issue where TimeMachine was complaining about not being able to backup to a USB attached external disk. (Even though there was plenty of space for the backup).

Apparently the connection was in an unexpected error state, so I did the following and all was working afterwards:

  • Shutdown the Mac
  • Unplugged the External HD
  • Unplugged the USB cable
  • Waited 60 seconds

Then plugged them back in and restarted the Mac.

All was good again. (Hope this works for you too.)

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  • Hello, thank you for the suggestion. I did all of that, but it didn't help. – tompave Nov 17 '19 at 23:45
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The first thing to try on Catalina is to tell it not to backup the system files and apps. Since the read only system is split, you will have a larger than normal new backup interval. You shouldn’t have to do this by my math either, but it’s low hanging fruit to try.

Open Time Machine preferences, select options and tick the box labeled Exclude system files and applications as shown below.

Exclude system files and applications If that gets you past the conservitave estimate, you’re set.


Next, I would just add a new drive - get whatever is less than $95 from a local store and set it up as a second destination. Leave both connected and the system will back up even/odd to each destination in turn.

Once you know you have a good backup, you can decide if it’s worth finessing the old backup or just let it be the history before Catalina and a place for manual backups of things like installers you download, second copies of photos and videos or just leave it on the shelf.

I use a tool BackupLoupe to analyze each drive in detail so you can see what needs cleaning up, but the time you spend trying to finesse this might be better spent unless you really want to get into the details of what changed on Catalina.

In that case - https://eclecticlight.co/2019/11/14/time-machine-has-changed-again-in-catalina/

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