Every day I notice several of my project DBs on SQL 2008 R2 (v. 10.50.2500) get into a limbo state where they are unavailable. They're not technically offline in this case. In Management Studio, there's no plus sign next to them to expand them, and when I right click > Properties I just get a single "General" pane with all data grayed out, with many properties just saying Unavailable.
We've found that we can correct this by going to Tasks > Take Offline then going back to Tasks > Bring Online
What could be causing this issue? Incorrect settings at the SQL server level?
Our SQL DB server runs on Windows Server 2008 R2 and the drives storing the DBs are on a SAN.
Your SAN probably has more than one RAID controller module. Typically when you create a virtual disk it will assign it to a preferred controller. If that controller becomes overloaded with IO requests, your virtual disk can be moved over to the non-preferred controller.
When this occurs SQL Server which is talking to the preferred RAID controller from host's iSCSI connection will notice that the database files are no longer accessible and take them "offline". This can also occur if the preferred RAID controller goes offline for other reasons.
To work around this problem you should configure an additional iSCSI connection/logon to the secondary RAID controller.