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

I am not sure all these convoluted cures are worth the time and effort, Apple just needs to get their act together and put out a patch. Until then I will use a laptop or iPhone if necessary.

I too have an iMac and I swapped out the hard drive for an SSD (hoping it might speed the ‘lemon’ up).

It is possible that the migration assistant step is where the Fusion drive is getting (re-)corrupted.

I have followed the steps in the diskutil resetfusion sequence and am in the process of setting up my iMac as a new machine. I’ll manually migrate my old local drive mailbox files.

I will report back on my results, though it will probably be a fairly long process (I’ve been living without the iMac since the problem occurred).

Since following the "diskutil resetfusion" sequence which is basically a complete fresh Catalina installation after resetting the Fusion drive (and not using the migration tool), my iMac and Mail app has been stable with no recurrence of the "mail cannot save" error.

6 days so far.

Great so if you have a day to waste doing all this (and still not a guaranteed fix) then fine, but Apple should be sorting this out. I have gone down those rabbit holes too many times in the past only to find it was pointless and am doing doing it again.

APPLE GET YOUR **** TOGETHER!

I did the following which reduced incidents considerably. Only had the error pop up once or twice but other than that, the system has been stable for several days:

Quit Mail, went to /Users/<username>/Library/Mail/V7/MailData/ and moved "Envelope Index" to the desktop (rather than deleting it). Restarted Mail and let it rebuild the index.

I don't think it's a perfect solution, but it's non-destructive and seems to have reduced the intermittent problem to maybe once or twice in the last 10 days. My Mac has not had to reboot since then.

Hope this helps! Good luck!

I just tried that and now it is even worse crashing all the time while rebuilding mailbox. Is this not an issue Apple should sort out?

I moved out folder back and now it just crashed every 5 mins, like before

Hi all. I had the same issues after updating to Catalina on my 2012 21.5 iMac with FusionDrive. This was up until yesterday when I had literally reinstalled a rest copy of Catalina, wiping everything from my HDD. Now, one thing I had noticed after upgrading to Catalina was that the when I went to Disk Utilities or View Storage under About This Mac I saw a "clone" of the HDD ("name of hdd - Data"). I had initially reinstalled Catalina but not doing anything to that "clone" and the issues persisted. My Photolibrary was double size, would not download from iCloud, mail crashed (with the messages from earlier comments), Lightroom would not install, etc. It was only yesterday when I wiped everything, deleted the "clone" and reinstalling Catalina that now the system seems to be working seemingly without issues or crashes. I DID NOT use migration or recover from back up. After installing, logged onto iCloud and also restored my documents (whatever was not on iCloud) one by one by entering into time machine.

Needless to say that before doing this a back up is needed.

So, check disk utility or storage information and if you have the same, a clean install, deleting the "clone" volume is the way.

There is hope! Good luck!

"...Now, one thing I had noticed after upgrading to Catalina was that the when I went to Disk Utilities or View Storage under About This Mac I saw a "clone" of the HDD ("name of hdd - Data"). .."

That is normal.

Catalina creates HDD as a read only drive for System files and HDD Data for User files. They appear to be the same size but they are not, one is not a clone of the other.

Thanks for the info. For a less technical user it certainly looks like some sort of duplication but I do believe it is done like you explained for a reason.

For me, it was only after I had erased that HDD Data prior to reinstalling OSX that I finally had a normal working system. Surely it must be linked.

I have the same issue with mail and photos. 2018 iMac in 2x to have the apple genus' work on it. I have a fusion drive and the issue only started after installing Catalina. Not sure what to do at this point but Apple needs to fix these machines.

Everything is working here still.

I believe that a key part of the so-far-successful process is the “diskutil resetfusion” (and to avoid using Migration Assistant).

“diskutil resetfusion” is NOT the same as "blowing everything away on the HDD" aka reformatting. “diskutil resetfusion” breaks apart and reestablishes the Fusion drive.

Turns out it is not a Mail.app problem at all.

It is a Fusion Drive issue that came bundled with Mac OS X Catalina.

Now I'm in the middle of a painful back-up all and completely reinstall afresh everything, after buying a 480GB SSD to back-up to before I die.

Hooray!

I have:

Late 2013

  Model Name: iMac

  Model Identifier: iMac14,2

  Processor Name: Intel Core i5

  Processor Speed: 3.2 GHz

  Number of Processors: 1

  Total Number of Cores: 4

  L2 Cache (per Core): 256 KB

  L3 Cache: 6 MB

  Memory: 24 GB

   1.12 TB Fusion Drive

My Fusion drive has: 300.8 GB (221.26 GB purgeable)

My mac is less than a year old, shouldn't Apple replace the fusion drive or fix Catalina or both? I had my macine in 2x and the Apple genus' didn't seem to know about the fusion/Catalina issue. Reinstalled Catalina each time and re-downloaded everything and still the problem persists.

I have just broken down the fusion drive back to it’s original ssd + hdd components.

now I’m installing El Capitan from the internet recovery onto the solid state drive only.

fusion no more.

120GB looks shy for a system drive but I have no other choice. Dang.