Why does my Mac HD have over 2 TB of purgeable data when I've only used 1.7 TB of regular data?

Good to hear that the purgeable space is going down!

The benefit of running First Aid from macOS Recovery is that Disk Utility can immediately start doing repairs on the boot volumes. When your Mac is started up normally, it can verify but not repair your boot drive. In order to perform repairs, the target volumes must first be "unmounted" (made unavailable to other processes, including macOS system processes). This is possible for external drives; however, unmounting the boot volumes isn't possible while macOS is running, because macOS depends on accessing those volumes.

macOS Recovery gets away with repairing basically whatever it wants, because it lives on a different volume (in fact, inside a dedicated image) than the typical boot volumes used by your Mac. To protect non-advanced users, Disk Utility doesn't show the hidden volumes that reside on your Mac. Those can only be revealed via the command line (Terminal).

To completely fix the purgeable space error, my best suggestion would be to back up all of your data, erase your Mac, and then import that data again using Migration Assistant (or when prompted to do so in Setup Assistant).

To completely erase your Mac running macOS Catalina or later (including macOS Big Sur), follow these steps:

1. Sign out of iCloud in System Preferences -> Apple ID. This ensures that Activation Lock is disabled.
2. Start up in macOS Recovery.
3. If you're prompted for an administrator password, skip to step 6.
4. The main window should appear. At the top of the screen, select Utilities -> Terminal.
5. Type this command and hit Enter (Return): resetpassword
6. At the top of the screen, select Recovery Assistant -> Erase Mac.
7. Follow the onscreen instructions to erase and reactivate your Mac. Your Mac will restart as part of this process, and might need an Internet connection to continue.
8. When the main window appears again, select Install macOS and follow the onscreen instructions.

Hi jdhoop,

Try checking your internal drive for errors, as described below. Disk Utility might be able to repair some errors it finds using the First Aid feature.

Considering the size of your internal drive and the amount of data stored on it, these steps may take a long time to complete.

1. Make sure all of your data is backed up onto a sufficiently large storage device (such as an external drive or a network solution).
2. Start up in macOS Recovery: Hold down Command-R as you power on or restart your Intel-based Mac. Release the keys when the Apple logo and a progress bar appear.
3. If you're prompted for an administrator password, enter it to continue.
4. The main window should appear. Select Disk Utility and click Continue.
5. Click the View button, then select Show All Devices.
6. Select the "Macintosh HD - Data" entry beneath the container entry:
   1. If a Mount button appears, click it. It should soon change to an Unmount button. If FileVault is enabled, enter your password when prompted to continue.
   2. If an Unmount button appears, you can safely proceed with the below steps.
7. Repeat step 6 with the "Macintosh HD" entry, beneath the container entry.
8. Select the Container entry and click First Aid. This will run First Aid on all of the volumes inside the container (including Macintosh HD and Macintosh HD - Data), and will then check the container afterwards.
9. During the check/repair process, click the arrow next to "Show Details" to reveal the log. Search this log for any notices pertaining to allocation or overallocation errors.
10. If any allocation/overallocation errors appear, please let me know and I can provide steps to fix those issues via the command line.
11. If any other errors appear in Disk Utility, check if they were successfully repaired. If they were repaired, run First Aid again on the container until no issues appear.