Set File Permissions so SQL can see external drives

If you want to host your databases on an external USB hard disk, it may be required to set the file system permissions on the drive so SQL can see it.

SQL uses a virtual account so its not always easy to set the permissions. To do this, go into the security tab and add a new user. Change the Location to the Local Machine, then enter the following name MSSQL$[InstanceName]

1. Using Windows Explorer, navigate to the file system location where the database files are stored. Right-click the file system folder, and then clickProperties.
2. On the Security tab, click Edit, and then Add.
3. In the Select Users, Computer, Service Account, or Groups dialog box, click Locations, at the top of the location list, select your computer name, and then click OK.
4. In the Enter the object names to select box, type the name of the per-service SID name listed in the Books Online topic Configure Windows Service Accounts and Permissions. (For the Database Engine per service SID name, use NT SERVICE\MSSQLSERVER for a default instance, orNT SERVICE\MSSQL$InstanceName for a named instance.)
5. Click Check Names to validate the entry. (If the validation fails, it might advise you that the name was not found. When you click OK, a Multiple Names Found dialog box appears. Now select the per-service SID name, either MSSQLSERVER or NT SERVICE\MSSQL$InstanceName, and then click OK. Click OK again to return to the Permissions dialog box.)
6. In the Group or user names box, select the per-service SID name, and then in the Permissions for  box, select the Allow check box forFull control.
7. Click Apply, and then click OK twice to exit.

https://msdn.microsoft.com/en-us/library/jj219062.aspx