Trying to install Catalina, running into problems of free space that don't make sense

[Hermolicious84]

Trying to install Catalina on a 2015 Macbook Pro that's currently running Mojave. It only has a 128GB SSD to I'm trying to clear some room so the install can happen.

Started by creating a bootable "Install MacOS Catalina" USB drive, so the install files aren't taking up space on the internal drive.

When I first boot into the Install drive and try to upgrade, it says that Macintosh HD needs another 35MB of space to install. Almost there!

Restart into Mojave, delete maybe 100MB. That should be good!

Boot back into Install drive, now Macintosh HD needs 200MB more space to install... huh?

Boot into Mojave again, and this time I find some synced Dropbox files that I don't need. Over 2GB worth! Turn off Dropbox sync, delete the files, and make sure the trash can is empty so everything is *really* being deleted.

Boot back into the Install drive... now it says that Macintosh HD needs another 500MB of free space to install. Seriously, WTF is going on?

I did notice that when I would go to "Get Info" on Macintosh HD along the way, the 'purgeable' amount kept going up. I ran a Time Machine backup, no change. At this point I have like 37 GB free and 17GB is 'purgeable'. The more I delete, the more free space it's telling me that I need in order to install. I don't know what to do.

 

[Fishrrman]

1. Back up anything you want to save to an EXTERNAL drive.
2. Boot to INTERNET recovery (command-OPTION-R). You'll need your wifi password, and it will take some time for the utilities to load. Be patient.
3. Once the utilities load, open disk utility
4. Go to the "view" menu and choose "show ALL devices"
5. Locate the "top line" on the left that represents the physical drive inside
6. Click on it and then click "erase"
7. Choose APFS with GUID partition format
8. Erase the drive and quit disk utility
9. Open the OS installer and begin clicking through for the install
10. The MBP will reboot one or more times, the screen may go dark for a few minutes, and you may see the "progress bar" appear more than once. BE PATIENT.
11. When the install is done, you should see the setup screen "choose your language". Begin setup.
12. At the appropriate moment, setup assistant will ask if you wish to migrate from another drive. YES, do this if you have a good backup (should be created with either CarbonCopyCloner, SuperDuper, or time machine).
13. Connect the backup and give setup assistant time to "digest" everything. Again, BE PATIENT.
14. I suggest you migrate everything
15. Let setup assistant do its thing. When done, you should see your login screen for your old account.

 

[Hermolicious84]

1. Back up anything you want to save to an EXTERNAL drive.
2. Boot to INTERNET recovery (command-OPTION-R). You'll need your wifi password, and it will take some time for the utilities to load. Be patient.
3. Once the utilities load, open disk utility
4. Go to the "view" menu and choose "show ALL devices"
5. Locate the "top line" on the left that represents the physical drive inside
6. Click on it and then click "erase"
7. Choose APFS with GUID partition format
8. Erase the drive and quit disk utility
9. Open the OS installer and begin clicking through for the install
10. The MBP will reboot one or more times, the screen may go dark for a few minutes, and you may see the "progress bar" appear more than once. BE PATIENT.
11. When the install is done, you should see the setup screen "choose your language". Begin setup.
12. At the appropriate moment, setup assistant will ask if you wish to migrate from another drive. YES, do this if you have a good backup (should be created with either CarbonCopyCloner, SuperDuper, or time machine).
13. Connect the backup and give setup assistant time to "digest" everything. Again, BE PATIENT.
14. I suggest you migrate everything
15. Let setup assistant do its thing. When done, you should see your login screen for your old account.

Can you explain why I need this crazy workaround solution?

What’s the core problem here?

Is there no other way to get rid of “purgeable space”?

 

[KALLT]

Can’t you just run the Catalina installer located on the thumb drive from Mojave? Perhaps the installer can help you out.

Perhaps there are still some APFS snapshots that prevent you from freeing up space:
tmutil listlocalsnapshots / (to see which ones are there, if any)
sudo tmutil deletelocalsnapshots / (to delete them all)

Normally, these snapshots are deleted within 48 hours. The Catalina installer should create a new snapshot right before it starts the installation.

 

[Hermolicious84]

Can’t you just run the Catalina installer located on the thumb drive from Mojave? Perhaps the installer can help you out.

Perhaps there are still some APFS snapshots that prevent you from freeing up space:
tmutil listlocalsnapshots / (to see which ones are there, if any)
sudo tmutil deletelocalsnapshots / (to delete them all)

Normally, these snapshots are deleted within 48 hours. The Catalina installer should create a new snapshot right before it starts the installation.

Once I'm running the installer via USB drive, it does not seem to have nuanced access to the internal SSD. It is treating the 17GB of "purgeable" space as used space that it can't touch. I will try running it from Mojave. For some reason I didn't think about doing it that way... I guess I've created bootable USBs in the past because I was doing clean installs, and I never considered just launching it inside the OS I was upgrading.

Thanks for the terminal commands. I was thinking they might be snapshots for time machine backups, but then they didn't change at all after a few backups, so I wasn't sure. Aren't the snapshots cleared when a backup is completed successfully?

 

[Hermolicious84]

Found an elegant solution

Someone recommended a terminal command that would create a huge file by writing zeros and fill all empty space on the drive, forcing MacOS to finally purge the unnecessary files it was holding onto. Purgeable space went from 17GB to <1GB in under 2 minutes. Installation started successfully.

 

[KALLT]

Thanks for the terminal commands. I was thinking they might be snapshots for time machine backups, but then they didn't change at all after a few backups, so I wasn't sure. Aren't the snapshots cleared when a backup is completed successfully?

No, the APFS snapshots remain there.

As you found out, macOS does have a mechanism to “thin” or remove snapshots instantly to recover space when it is needed, such as when writing a lot of data. You could have achieved the same result with the last command I listed.