I am having abc.mdf database in D drive and i want to use in my website using web.config?

I am having abc.mdf database in D drive and i want to use in my website and i have tried as in web.config

and i got following error

An attempt to attach an auto-named database for file D:\abc.mdf failed. A database with the same name exists, or specified file cannot be opened, or it is located on UNC share.

I am sure database doesn't exists anywhere else.Please help

Please make sure "vik.mdf" file is not already placed in MSSQL Data folder (C:\Program Files (x86)\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\MSSQL\DATA) or database is not already attached in Microsoft SQL server Management Studio.

When you report that "If I am using User Instance=true with above it raises error... The user instance login flag is not supported on this version of SQL Server" it sounds like you aren't using SQLEXPRESS. Only SQLEXPRESS supports user instances.