Time machine backup disk can't be unlocked

I have dual backups to external SSD. This morning after a system restart I got this error message "The disk MDBKb can't be unlocked." I can run Disk Utility First Aid on the ssd, it finds no errors. Cannot run First Aid on either container or Time Machine partition [the whole disk] because it will not mount. I would appreciate any thoughts folks have on how to get this operational again. I'm wondering now about the stability of the other Time Machine ssd. both are Samsung PSSD T7