Offline and separate the difference between:
Separation and offline can make the database can no longer be used, but after the separation to require additional use, but only after the online offline can be used.
Additional database error:
Unable to open the physical file XXX.mdf "operating system error 5:." 5 (Access Denied.) "(Microsoft SQL Server, Error: 5120).
Solution:
Xxx.MDF found files with the file and folder where xxx_log.LDF
Right - Properties - Security - in group or add the user name Authenticated Users
Change the permissions for the group full permissions, additional success again.