Access is denied when attaching a database

Run SQL Server Management Studio as an Administrator. (right click-> run as administrator) that took care of all the weirdness in my case.

SQL SRV EXPRESS 2008 R2. Windows 7

Thank you for all of the comments. Some of you helped to lead me to the answer. Here's what I found:

It was an NTFS permission problem, and not a SQL problem. Further, it looks kind of bug-like (and it's repeatable).

The problem:The account that I was using had full control NTFS permissions to the mdf and ldf files. However, it had those permissions through group membership (the Local Administrators group had permissions, and my account is a member of local admins). (I verified the permissions)

If I try to do the attach, connect to SQL Server as me (where I am in the admins group), it fails with the NTFS problem.

However, if I grant the same file permissions that the local admin group has directly to my Domain Account, then I can attach with no problems.

(oh, and yes, I checked the local groups on this machine, and I verified that my domain account is indeed a member of the local admins group).

So, it looks as though the error occurs because some code (either in SQL Server or Management Studio) checks for the permissions that the user account holds, but it doesn't go so far as to check group permissions that the user account inherits.

That sounds weird to me, but I can reproduce it over and over again, so I have concluded that it is the answer.

Update: I reported this as a bug: https://connect.microsoft.com/SQLServer/feedback/details/539703/access-denied-attaching-a-database-when-permissions-are-inherited

I'd like to add additional info to the answers that were posted.

Be careful when detaching the database because the windows user you are logged in as becomes the only user with permissions to the .mdf file! The original permissions the .mdf file had which included the user SQLServerMSSQLUser$<computer_name>$<instance_name> and the Administrators account get overwritten by whichever windows user you are logged in as (not sql server user). Boom, all permissions gone just like that. So do as others have said and right click your .mdf file and double check the permissions.

I ran into this problem because I used SSMS to connect to the database (doesn't matter which sql server account) and detached the database. After doing that my windows user was the only one that had any permissions to the .mdf file. So later on when I tried to attach the db using the sa account, it threw the "access denied" error.

To keep the original permissions in tact you should take the database offline, then detach, then attach in that order like so:

USE [master]GO-- kick all users out of the dbALTER DATABASE mydbSET SINGLE_USER WITH ROLLBACK IMMEDIATE GO-- Take the Database OfflineALTER DATABASE mydb SET OFFLINE WITHROLLBACK IMMEDIATEGO-- detach the dbEXEC master.dbo.sp_detach_db @dbname = N'mydb'GO