Mail cannot save information about your mailboxes because there isn't enough space in your home folder

Hey again, sorry that didn’t work. Can you remember step by step how you reformatted the drive? Which disk was erased (Or Volume), and which format etc?

And can you confirm if you do have a Fusion drive or not?

You can check: Apple > About This Mac > System Information > Storage, as in:

https://support.apple.com/en-us/HT207584

Thanks! Sorry all the hard work hasn’t yet paid off, hang in there!

Hey there! Agree with Barney-15E. At first you’d think a storage issue, then a possible permissions issue with the user account. I think it might affect fusion drives somehow that then require them to be rebuilt/reset. Whatever the case reformatting the drive seems to fix the issue. Hope it works out!

Are you migrating from another Mac, an external drive, or a network drive? (Like a Time Capsule, etc). 25 hours is excessive for sure.

You May need to set it up as new, then move what you need back onto it manually, (Using Finder).

No worries, yeah, sometimes it’s a good idea to just not migrate the data or restore the backup, it might be more work, but if it’s taking that long you might not want ALL of that data coming back on again. Hope it works out, cheers!

Oh good! So was the drive split and you guys had to do the diskutil resetFusion command? Or did it look normal, in my case I saw the HDD and SSD were kind of backwards so I had to run that to fix it, (Much easier than it used to be by the way, thankfully). Either way glad it’s sorted out!

I erased the drive, reinstalled Catalina, then migrated from my Time Machine backup during the Startup Assistant.

I found no other solution to the problem.

No, it shouldn't take that long. It does take several hours to do the migration depending on the amount of data.