I have a Seagate 3TB external drive that used to work.
I've just upgraded my Mac to Sonoma 14.1.2 (23B92)
And the disc won't mount. No errors. Verify is OK. Repair is OK.
Update, I followed the troubleshooting guide and a reinstallation of the drivers seemed to fix the issue.
Just need to sort out the license (I had a free one with the Seagate drive) - raised a support call.