I'm having the same problem.
Server 2012 standard edition
SQL Server 2012 enterprise edition
a domain service account, which is also in the server's local administrator group
the domain service account is a sysadmin in MSSQL
SQL Server is in mixed mode
when I run the MSSQLSERVER service as the domain service account, I can only see the C drive from within MSSQL
when I run the service as Local System account, I can see all of my drives
It 'seems' that things run as local system account, but I "need" it to run as a domain service account so I can use ALWAYSON.
Any thoughts or ideas? I'm at a loss.