MySql service on Windows won't start after changing datadir
If You open Windows Services and check the service properties, You will find something like this:

"C:\Program Files\MySQL\MySQL Server 5.7\bin\mysqld.exe" --defaults-file="C:\ProgramData\MySQL\MySQL Server 5.7\my.ini" MySQL57


not enough to change datadir in my.ini file, need also point windows to run service with proper my.ini file