Big Sur Mail App Brokenness

Greetings,

I appreciate that but the issue was far more complex. The 9.8 GB turned out to be inside of ~/Library/Mail, but I didn't have any access to it until I: deleted Mail app and then re-installed macOS Big Sur to force the system, to recreate Mail.app and associated directories.. My theory is that is must have been in some other weird location and that Big Sur was smart enough to fix it all. In seems such code would be pretty simple actually. :)

Here are the instructions to repair this for anyone also experiencing this issue (and perhaps losing their minds):

<START>

1) The phantom data was ultimately linked to *~/Library/Mail* (*if you have this, search there and also ~/Library/containers Mail*, and anything else in there with MAIL in name.  

2) However, I *didn't have *~/Library/Mail* in this location, but may have it during a previous install of macOS. For me at least, it seems safe to blame this on *"Big Sur Unique upgrade or re-install wonkiness"*. My current theory is that this storage must have been somewhere else and the re-install of macOS simply re-organized it into the proper directory structure. 

3-A) Reboot into System Restore Mode (CMD-R on boot-up for  Intel; Hold power-button boot-up on Silicon)  

3-B) Utilities-->Terminal: <NO-"QUOTES-TO-THE-RIGHT-OF-THIS">  "csrutil disable" (this disables*system integrity protection*) which is re-enabled at step 3-D.  

3-C) Start Mac normally-->Terminal:   "cd Applications" <enter> then type "sudo rm -rf Mail.app/"

3-D) As you have been expecting, GOTO 3A... csrutil enable.   

4) Now MAIL is deleted and won't have an entry in *Manage--> Storage*, but is still on local machine.  

5) System Recover again--> *Reinstall macOS Big Sur*  

6) Now, you WILL have *~/Library/Mail*, likely with a large amount of folders and subfolders inside.  

7A) With *Manage-->Storage* open for viewing, expand tree of *~/Library/Mail* and start deleting things from innermost structure. You will notice that the storage designation of MAIL will start to trickle away. :)   

7B)  If you delete any main folders, and certain subfolders, like contents of <DRAFTS>, the MAIL app will disappear from *Manage Storage*. You can SHIFT-DELETE to get it back.    

8)  But if your intention is also to terminate your relationship with MAIL, here is my recommendation:   

Have some fun manually deleting folders and watching the previously-unknown file size slowly deflate. 

Simply delete the entire folder *~/Library/Mail* and rejoice that your *WONKY-MAIL-STORAGE-BECAUSE-OF-BIG-SUR* issue has been resolved!  

QUICK FOLLOW UP: For anyone reading this, I will share: several friends are emphatically insisting that ~/Library/Mail MUST have been on my system before I went through this exhaustive process. I checked a Time Machine backup prior to this work, and it is not. I don't know if it's possible for something to exist in a directory tree but not appear visually (ie- it could be hidden)(?) Additionally, it seems relevant to add: on this newish MacMini M1, I had re-installed from backup once (after messing up Python via Terminal play), following the new "migration assistant" process. This is bizarre (Big Sur users can not longer use Time Machine in the same manner as previous os's) as you need to create a brand new user, which I did. Both of these users were in various states of iCloud around this time, and I eventually deleted one of the Users. Perhaps this contributed to said wonkiness.