Cut and pasted this from another website. Works a treat and was exactly what I needed.
First of all, you’ll need to be in the Steam beta. To get there you need to run Steam, click on the file menu option Steam, then pick Settings. In there, you’ll see “CHANGE…” under “Beta participation”. Click on that, and opt into the beta. It’ll automatically restart, but when it’s done you’ll want to quit out completely.
Now you need to add a switch to the Steam shortcut. So find the icon, Start menu option or whatever shortcut you use to load it, right click on it, and choose Properties. In there you’ll see the path it loads from looking something like this:
C:\Steam\Steam.exe
You want to add “-dev” to it, after a space, so it looks like this:
C:\Steam\Steam.exe -dev
Run that, and now you’ll see an extra page option, on the right after NEWS, called CONSOLE. Click on that one, and then type the following into the text field:
install_folder_ui
Hit return.
Choose “Add install folder”, and pick another place where you’d like to install Steam games. And you’re done!