One of my start-up drives is missing
Installing Yosemite converted your hard drive to a Core Storage logical volume, and CS was only introduced in Lion (Wikipedia), so it makes sense that Snow Leopard would have no idea what to do with it. Here's how you undo CoreStorage (source - this method worked for me). It should be non-destructive, but as always, take a backup first! 

Also, note that any reinstall of Yosemite will re-Core Storage-ify your drive and require you to do this procedure again, and software updates might too. If the issue comes back after a reinstall or update, just go through this process again.


Boot into Recovery HD. If for any reason you don't have Recovery HD, then you'll have to make a bootable Yosemite drive and boot into that.
In the Menu Bar, select Utilities > Terminal.
Run diskutil cs list. You'll get an output that looks like this (credits to the Ask Different user in my source):






Select your Logical Volume ID (the one highlighted in the screenshot) and copy it to the clipboard.
Run the Terminal command diskutil cs revert Paste-Your-Logical-Volume-ID-Here.
Run diskutil cs list to verify that your computer can't find any CS volumes.
You're done! Now Snow Leopard should be able to recognize your SSD's potential as a startup drive again!