So... I wrote this
Basically, just extract this small script and save it to somewhere handy and/or create a shortcut. Edit it to change the location of your current Steam cache directory, and the path to the other drive/directory you would like to use to use as your "alternate cache". If there are spaces in your directory names make sure to put them in quotes. So you can leave your Steam cache in the default location on your "fast" C: and set the alternate cache to your slower D: drive. All the games will still work regardless of which place they are in.
What does it do?
Essentially it moves the chosen game directory to the alternate cache location and then create a "junction point" in the original location that points to the new location. Anything looking in the original spot for the files doesn't even know they have moved and they still "appear" to be in the same place.
- You install Steam to the default location on your C: drive. This is where you have your OS installed and would be the "fastest" drive in your computer.
- You have another larger (but possibly slower) drive installed D:
- On the D: drive, create a directory D:\steamcache
- Edit the script to specify "D:\steamcache" as your alternative cache
- Run the script and type the name of the game directory you want to move to your D:
- Only the game directory you select will be moved, so you can choose to keep your larger and most played games on the fast drive and move all the others to the "slow" drive
This can also work in reverse. For example if you install a new faster drive as D: and without wanting to reinstall Windows and apps etc. You can instead just move the games you play most often to the alternative cache location.
There are a bunch of checks in place to make sure you can't do something wrong, but I can't test every possible scenario, so use at your own risk. I've tested it on a few of my Steam games and they all still work fine.
Any questions, let me know.