Catalina: Time Machine claims there is not enough space for a backup
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.)
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.
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/