Unable to add db_datawriter to user defined role I want to grant a user defined role, read and write permissions to the entire database and add users to it. I was unable to do this using GUI. So I tried the solution given at the link - https://www.sqlservercentral.com/Forums/Topic1141208-1526-1.aspx but it gives me the error mentioned below when I try to add db_datawriter to the role I created. 

Cannot use the special principal 'db_datawriter'

The command I executed - EXEC sp_addrolemember 'DR_User', 'db_datawriter'

Please help me figure out the right way to achieve this.
JKay, you are a little mistaken about the syntax of this command:

EXEC sys.sp_addrolemember [ @rolename = ] 'role',  
    [ @membername = ] 'security_account' 


So in your particular case the command is:

EXEC sys.sp_addrolemember @rolename = 'db_datawriter', @membername = 'DR_User';