Thank you both for your prompt replies.
It looks like whatever the “upgrade” process did at least renamed the original mysql directory under /var/lib to mysql.bak which seems to contain a few sizeable files.
running systemctl start mysql fails with the following error:
> Job for mariadb.service failed because the control process exited with error code. > See "systemctl status mariadb.service" and "journalctl -xe" for details.
Running journalctl -xe shows:
Jan 10 23:36:28 osmc mysqld[2421]: 2018-01-10 23:36:28 1996288000 [Note] Using unique option prefix ‘key_buffer’ is error-prone and can break in the future. Please use the full name ‘key_buffer_size’ instead.
Jan 10 23:36:29 osmc mysqld[2421]: 2018-01-10 23:36:29 1996288000 [Note] /usr/sbin/mysqld (mysqld 10.1.26-MariaDB-0+deb9u1) starting as process 2421 …
Jan 10 23:36:29 osmc mysqld[2421]: 2018-01-10 23:36:29 1996288000 [Warning] Can’t create test file /home/osmc/.mysql/osmc.lower-test
Jan 10 23:36:29 osmc mysqld[2421]: [93B blob data]
Jan 10 23:36:29 osmc mysqld[2421]: 2018-01-10 23:36:29 1996288000 [ERROR] Aborting
Jan 10 23:36:29 osmc systemd[1]: mariadb.service: Main process exited, code=exited, status=1/FAILURE
Jan 10 23:36:29 osmc systemd[1]: Failed to start MariaDB database server.
– Subject: Unit mariadb.service has failed
– Defined-By: systemd
– Support: Debian -- User Support– Unit mariadb.service has failed.
– The result is failed.
Jan 10 23:36:29 osmc systemd[1]: mariadb.service: Unit entered failed state.
Jan 10 23:36:29 osmc systemd[1]: mariadb.service: Failed with result ‘exit-code’.