Backup failure for a DB part of Always-On Availability group - SQL 2012 Ent Edition We have a DB that we are unable to take full backups of or able to join it in AG group. The T-logs are getting succesful backups. The DB is stuck in doing a CHECKPOINT that is blocked by a DB STARTUP process. Is there anyway to clear this internal process without having to reboot? It has been sitting like this for a few days. We are able to restore the DB using the T-logs but the DB is not highly available due to not being part of the AG. We are on SQL 2012 Enterprise edition. Please don't give me an answer that I need to upgrade to latest version. 

Has anyone experienced this issue in SQL 2012 or any other version when using AAG?
MS discovered that this is an issue with 2012 CU3. We ran into this bug: https://support.microsoft.com/en-us/help/3033492/alwayson-availability-groups-may-be-reported-as-not-synchronizing-afte   

This problem is resolved in SQL 2012 SP4.

In the interim - the solution is a reboot to get the DBs off the bad state.