If you want to move your game onto another drive.
First of all there is a verry good help from steam itself. Open steam and there you can hit help and search what you are looking for.
It explains well how to do.
Here is the link:
https://support.steampowered.com/kb_...7710-TDLC-0426
and
https://support.steampowered.com/kb_...9609-OBMP-2526
LC