I already Win 7pro installed, I'm trying to install Win 10 pro. It won't let me partition more than 40 GB's out of the 210 GB I have free and I don't know why.
First off, 40Gb is plenty enough for the OS as long as you install any programs to a different drive.
Second you will have to defrag your drive with a defragger that will allow you to move all the data to the front of the drive, drive manager only allows you to create partitions as large as the free space, so if you have random files up to 40Gb before the end of the drive then 40Gb is the most you will get.
Third if you want to save yourself a lot of hassle and danger of losing all your data and/or partition setup, yust create a virtual hard disk from within windows 7 disk management and install windows 10 on that and add it to the boot menu.It's called windows to go if you want to look up a guide.