Same problem iMac 2017 date mid June 2021; RESOLVED issue this time doing a Disk Utility First Aid run then restarted - magic T5 suddenly mounted after multiple other attempts.
Sandisk Extreme SSD would not mount on 2016 macbook pro or on new 2021 macbook M1. Disk utility would not mount. FYI, You cannot run Disk Utility First Aid without mounting the drive, so don’t understand why some users report this as a solution to the drive not mounting. Resetting PRAM had no effect. RESOLVED: Disk Drill for Mac mounted the drive without issue. Once the drive was mounted in Disk Drill it appeared as mounted in Disk Utility. At this point it was possible to run First Aid on the disk. Exit code on the disk was 0, disk repair corrected the error and now all is good, for now Hope this helps.