Fingers crossed, I have not yet experienced the dreaded error message "Mail cannot save information..." after repeatedly encountering this error message after various attempts to solve the problem.
My punch line solution was moving a large number of high volume video files that were recently created and managed in the past six months to an external SDD drive.
What does moving large files from my fusion drive, not related to my mail app, have to do with this error message? I cannot definitively answer that question. However, after receiving suggestions to reformat fusion drives and given my methodical nature of initially implementing small changes in an IT environment, I started looking at literature on the fusion drive before I, as stated perviously, perform a lobotomy on the fusion drive.
Please excuse me if what follows is pedantic. Apple’s fusion drive combines traditional hard disk and SSD storage in a drive that appears to the operating system as a single volume. Hard disks provide a lot of storage for a low cost per gigabyte, but they’re relatively slow. SSDs — solid-state drives — are much faster but can cost more per gigabyte. By fusing the hard disk and SSD into a single volume, a Fusion Drive provides a lot more storage than an SSD with far better performance than a hard disk. macOS achieves this performance by storing operating system files, commonly used apps, and your most frequently used data on the SSD. Adjusting data storage locations happens behind the scenes, so you don’t (and can’t) know what data is stored where — some files may even be stored in both places.
The Fusion Drive, introduced about nine years ago, relies on a fast SSD and a slow (5,400 rpm) HDD, and optimizes the SSD storage to hold the most frequently used data. This can allow fast boots and keep apps running quickly. Which is great!
Drive manufacturers that offer hybrid drives often embed the SSD storage into the same package as the HDD. Apple, in contrast, puts an SSD on the computer motherboard separately from the HDD, and relies on macOS to integrate the two. (Perhaps this is where comments arise about the error message cropping up with new iterations fo the OS.) Files aren’t stored separately on the two drives, but rather macOS interleaves data so that it’s effectively like one big drive.
That’s great for performance and cost, but it’s highly problematic if your HDD fails or if your Mac bites the dust or you encounter performance problems with apps…. and if you have to be able to recover data from both the HDD and SSD, including removing both of them physically from a Mac in the case of device failure, to recover the data as a whole. Otherwise, it’s like trying to put a multi piece puzzle together in which it’s not like 32 pieces are missing, but small parts of hundreds of pieces can’t be found.
I read somewhere that the 32 GB SSD in the current 1 TB Fusion Drive is actually an improvement because in 2015 Apple reduced the size of the SSD from 128 GB to 24 GB. I have a 2013 27in iMAC with a 3.2 GHz quad core processor, 16 GB 1600 MHz DDR3with a 1.12 TB fusion drive. I don't know how much of the drive is SSD put I am thinking it is 24 GB. I also don't know what mix of files (data, operating and applications) reside on the SSD. I also don't know how the current macOS Catalina 10.15.3 manages the SSD in terms of placing 'frequently used files' on the SSD to improve performance.
So, I thought, one initially minimally invasive approach to this problem would be to move large data files (video files) that I have used in the past year off of the fusion hard drive and onto a separate SSD drive. Once I moved the files, I deleted the initial copies off of the fusion drive.
I have not encountered mail app error message since. Is this fortuitous? I don't know. But similar to many things with the MAC operating system and architecture, it is more of a challenge to tinker 'under the hood' than it is in a Windows or freeware operating systems.
