I don't post much and usually when I go on to forums I am the one asking for help. I had a similar issue where I updated to Unraid 6.9.0 and after I rebooted, Disk 5 was not found. I tried several reboots but to no avail. Eventually, I powered down the system, opened up the server chassis, unplugged Disk 5's data and power connection, re-seated the LSI 9211-8i expansion card. Started it back up and viola! Disk 5 available again. Not sure if the update caused it, but decided to post my solution in case someone else has the same issue.