HERE! M2 Pro Mac mini. I have an external APFS formatted ADATA M2 2TB external ssd in Xcellon USB C 3.2 enclosure (NOT encrypted). Worked perfectly until the update to Sonoma. I was concerned that the update somehow corrupted something, but is certainly not the case as the drive and usb cable still work as expected on other non-sonoma systems i've plugged it into.
After the update, the drive in question (which is the only one of my two external drives having problems, more info below) stopped mounting, with a window popping up (will get a screenshot later) saying that it could not mount, and some options which included ignoring, and initializing anyway. Clicking on initialize does nothing. Diskutil and Disk Utility see that a device is connectd, but cannot read any info from it, and it shows a capacity of 0K. One attempt at a reboot did result in the drive being mounted for a few seconds, long enough that I was able to navigate to the drive in finder and see the data i have there. Shortly after, unmounted and have not been able to do anything about it since.
I am discouraged in that among the subset of people who are experiencing this, I dont know how many also have un-encrypted drives, and as such i'm not sure the proposed solutions are applicable.
Oddly enough, I have a samsung T5 external ssd, with an encrypted partition for time machine, and an unencrypted partition for additional data, that IS working as expected, upon reboot i need to enter the encryption password, and then mounts fine. Did not expect this kind of err on the part of Apple when it comes to their OS.
Sonoma certainly broke something for some of us, and I hope this is taken seriously.