Like many, I had this problem - my particular set up is as follows -
External Drive : OWC Aura P12 Pro PCIe 3.0 NVMe M.2 2280 SSD with 2x 4TB ram cards
Mac : Mac Studio M1 Ultra
OS : Sequoia 15.3.2
Had a Bad Idea and decided to clean up the major mess behind the big screen of tangled cables, USB hubs, power supplies, external drives, a mini Tardis machine etc etc - your basic horror show from plugging in every next device in, without ever rationalising any of it.
The OWC SSD had worked flawlessly since installation in June '23 - and just used any old generic thunderbolt cable to connect it and all worked fine.
After untangling and losing a *LOT* of excess and not often used gear, everything booted back up fine -
Until the Sequoia update ...
The OWC no longer appeared in either Finder or Disc Utility, which was weird because it had been a rock solid always there device for 1.5 years. Because there were no directories or files visible from the OWC could not use for backup or as an extension for big apps of the Mac's 1TB hard drive - a big drama!
For the same reason could not use the right click trick on any OWC files or folders in Finder, followed by Get Info / Sharing Permissions to alter the Permissions for the OWC.
Strange how often the basic 101 support solutions sometime work?
Changed the Thunderbolt cable between the Mac and the OWC to a "genuine" Apple Thunderbolt white job (not sure what it came with) and hey presto OWC + all folders and files appeared in both Finder and Disk Utility.
Apple are (inconsistently) picky about what, and for how long, 3rd party devices can be used with their gear, despite not making a whole bunch of external devices they know you will need over time (and they often sell those third party devices in their stores).
Morals -
- Keep a set of genuine apple cables around to test when you have a non connect issue (I know that 3rd party ones are often better made, cheaper and in more convenient lengths)
- Before upgrading any OS on any device, wait and search known issues before upgrading, especially compatibility and support issues.
Just feel a bit silly reporting such a simple fix .... cheers Max