Been having this issue with he same 49218 error appear on several external SSD's that were disconnected without a proper unmount. In the past doing ejects and unmounts from the terminal allowed the drive to reconnect but no luck this time around. You can see in the screenshot the OS can see my partition but it will not mount, the greyed out partition. I am now at a loss as well, no idea how to continue to troubleshoot. Seeing as I've seen this issue on multiple drive's over the past year or so, Apple needs to get it together and fix. Unmount, eject and trying to remount does not work, first aid does not work.
bfeinz
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