Unable to open the physical file "C:\x\a.mdf"

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C#3.5

Hi
how to fixed the under error :

" Unable To open physical file : " c:\x\a.mdf".operating system error 32:32(error not found")". an attemp to attach auto-named database for file c:\x\a.mdf failed. A database with same name exists, or the specified file cannot be opened,or it is in located on UNC Share"


my sqlconnection as :

using (SqlConnection con = new SqlConnection(u.connectstr))
               {
sqlcommand cmmd = new sqlcommand (x,con)
...
}

one of solution that I use for solve above error, copy project file to the another folder. but It is bad and incorrect !!!???