I have had this error on my 2017 mid 5k iMac 27in. As another posted if you run the diskutil-verification tool you shouldn't see it BUT that does not excuse apple for not addressing this problem. The amount of $ we all spend you would think someone would of the common decency to post SOMETHING about this MT-Mapping problem that has been happening for two years now. Welcome to customer server 2021. No CSR's answer you and they have search engine algorithm's so jacked up because of the suppression tools it makes a simple search turn into a ready to throw out the window situation.