One of my start-up drives is missing I have a Mac Pro (Early 2009) in which I am running two OS's.
10.10 on a Mercury Accelsior SSD and Snow Leopard on a conventional drive. Before upgrading the SSD from Mavericks to Yosemite all was hunky-dory. I could restart between the two OS's using the start-up drive selector in the system folder. Now I have gone to 10.10.3; when I switch from Yosemite to SL and restart from the conventional drive I cannot see the SSD Yosemite Drive in system prefs only the Snow Leopard drive is visible (and the network option) 
Restarting FROM SL and resetting the PRAM eventually brings me back to Yosemite.
OR
using the sudo bless command -mount /Volumes/"name of 10.10.3 start-up drive" -setboot
followed by a sudo shutdown -r now
also gets me back into 10.10.3
However I dont like doing this as it feels rather the same as pulling the plug; the Mac is not going through it's correct shut-down process.
A long conversation with Apple arrived at the suggestion I should re-install SL something I do not want to do - too drastic + I may not be able to get back to what I have.
So two questions:
1/. is there a 'gentle' way in terminal I can 'command' the mac to restart from the SSD
and
2/. Is there a way I can 'replace' the start-up info for SSD
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!