CheckDB error because snapshot file already exists

A ordinary Maintenance Plan with a ordinary Check Database Integrity task failed with this error in the Maintenance Plan log:
A database snapshot cannot be created because it failed to start.
Cannot create file 'D:\\Data\\MyDatabase_Data.mdf_MSSQL_DBCC75' because it already exists. Change the file path or the file name, and retry the operation.
The error text indicates that the file exists which is true.
Also the text indicates that the file path or name can be changed. This is not true in this case as one can't define the physical database snapshot when running a Check Database Integrity task. This is why it can't be defined when running DBCC CHECKDB statement.
In the SQL Server Errorlog the failure is also logged:
2015-08-16 03:11:19.77 spid42      Error: 5170, Severity: 16, State: 1.
2015-08-16 03:11:19.77 spid42      Cannot create file 'D:\Data\MyDatabase_Data.mdf_MSSQL_DBCC75' because it already exists. Change the file path or the file name, and retry the operation.
As the information is similar the failure is confirmed on file details.

In this case the server had a hard shutdown (ups!!!) which caused the CheckDB failure.
To confirm if the database snapshot on the file is still active this statement:
SELECT *
FROM [master].[sys].[databases]
WHERE
  [databases].[name] = 'MyDatabase' AND
  [databases].[source_database_id] IS NOT NULL;
The result is empty which is confirmed by the message:
(0 row(s) affected)

The snapshot file can be inspected on the metadata through PowerShell using the CmdLet Get-Item:
Get-Item -LiteralPath 'E:\Data\MyDatabase_Data.mdf_MSSQL_DBCC75'

As I found no other reference on this error I preserved the database snapshot file by moving it to another location.

Testing the solution is quite simple as I only have to run a CheckDB on the database:
DBCC CHECKDB ('MyDatabase');
As planned the run is good:
CHECKDB found 0 allocation errors and 0 consistency errors in database 'MyDatabase'.
DBCC execution completed. If DBCC printed error messages, contact your system administrator.

Attach database

I had to get some data from a old database, where the server is recycled. But at that time I was present enough to get a copy of the data files of the database (mdf & ndf).
The quick solution is to attach the database on my workstation where I have a SQL Server Developer Edition running. This statement should do the trick:
CREATE DATABASE [Repository_old] ON PRIMARY
  (FILENAME = 'C:\Program Files\Microsoft SQL Server\MSSQL12.MSSQL2014\MSSQL\DATA\Repository.mdf'),
  (FILENAME = 'C:\Program Files\Microsoft SQL Server\MSSQL12.MSSQL2014\MSSQL\DATA\Repository_2.ndf')
FOR ATTACH_REBUILD_LOG

But when I tried to attach the database I got a error on file access.
Msg 5120, Level 16, State 101, Line nn
Unable to open the physical file "C:\Program Files\Microsoft SQL Server\MSSQL12.MSSQL2014\MSSQL\DATA\Repository.mdf". Operating system error 5: "5(Access denied.)".
A quick look on the other SQL Server files in the same folder indicated that the service account should be owner of the files. The error message is on the first file in the statement, but this goes on all files for the database.
This can be done in Windows Explorer through the properties of the file (right-click > properties). On the Security tab you click the Advanced button. Then you get another window named Advanced Security Settings for ... In this windows Owner tab you click the Edit button. Now you might have to log on as you are required to be local administrator.
Click the button Other users or groups to enter the service account. This is not straight forward if you are using a virtual service account. First you have to pick the computer as location. Then you can limit the object type to Builtin service accounts.
Finally entering the name of the service account is a little tricky - in my case I entered "nt service\mssql$mssql2014". Notice the space after "nt"!
I am working on the named instance MSSQL2014 which gives the name of the account after the backslash.
Before clicking the OK button I do recommend that you verify the account name by clicking Check names. If the name is changed to capitals, the "nt service\" is removed and the name is underlined it is OK.

The service account is (default) a Virtual Service Account. This is a local edition of a Managed Service Account. To get the name of the service account you can look in the Server Facet of the instance with Management Studio.

But still I got error on file access.
Msg 5120, Level 16, State 101, Line nn
Unable to open the physical file "C:\Program Files\Microsoft SQL Server\MSSQL12.MSSQL2014\MSSQL\DATA\Repository.mdf". Operating system error 5: "5(Access denied.)".
I usually work on installation where DBAs are isolated from other admins. In this case the database files are moved/copied around using one account and the attach statement is executed with another account. When I then added the user of SQL Server Management Studio (SSMS) who executes the statement to the files with Full Control permissions everything went smooth.
I first tried to limit the rights for the SSMS user the Read permissions, but that didn't work as I got this error
Msg 1813, Level 16, State 2, Line nn
Could not open new database 'Repository'. CREATE DATABASE is aborted.

In this case I did not have the transaction log file (ldf), but I really didn't need it. This is why the statement is with FOR ATTACH_REBUILD_LOG. There are some issues you have to consider like a clean shutdown of the database. Please take a look at the documentation for CREATE DATABASE.
The original transaction log file could not be found by the instance, as expected. This generates a failure message like
File activation failure. The physical file name "H:\MSSQL\Data\Repository_log.ldf" may be incorrect.
But right after the new transaction log file is created automatic
New log file 'C:\Program Files\Microsoft SQL Server\MSSQL12.MSSQL2014\MSSQL\DATA\Repository_old_log.ldf' was created.