When trying to reattach a database I kept getting following error:

In the SQL log I could find below error. The same error was given when trying to attach the DB with a T-SQL statement.

Msg 5120, Level 16, State 101, Line 3 Unable to open the physical file “E:\Program Files\Microsoft SQL Server\MSSQL10_50.BTSISDEV1\MSSQL\DATA\BizTalkMsgBoxDb.mdf”. Operating system error 5: “5(failed to retrieve text for this error. Reason: 15105)”.

After several attempts on how to solve this I finally found the solution.
Turned out, all I needed to do was run the SQL Server Management Studio as an Administrator.
How can it be that simple, yet has me looking for a solution for over an hour :).