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.