1. If we want to switch to latest FD 7.0 then you must need to update the Latest Service Pack of 6.1 version first and then switch them to the latest FD 7.0 and respective Service pack.
2. If we upgraded the software to FD 7.0 and getting the below mentioned error at the time of backup, Then you need to follow the below step.

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Solution: Open Registry Editor >> Type "SQL_version"  >> modifty >>

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Try to take a backup again and it will not gives you the error anymore.
(Note: FD 7.0 is SQL 2014 based that is why we need to change this registry setting)