If i have windows 8.1 installed on an ssd, and 2 striped data drives, and an open unused drive
1) ssd OS
2) data drive 1
3) data drive 2
4) unused drive
if i were to remove 1, 2, and 3, install steam os on 4, then put 1, 2, and 3 back after the installation
would the first three drives work just as they used to? and remain separate from the steam OS?
Or you could just throw your Windows Steam into auto start in Big Picture mode and save yourself the headache. If you have a working Windows computer with Steam installed, you get all the same features that SteamOS will include. But you also get to keep your Windows game library as well. SteamOS isn't really meant for dual booting, it's meant as either it's own standalone operating system -or- a supplemental extension of your desktop ( streaming to a local computer elsewhere, such as the living room as Steam has used for all their advertising ) free or charge ( not required to buy a $100 Windows license ).
Now, if you don't mind losing your Windows games just to install SteamOS for no real reason whatsoever, then to answer your question yes. But I would honestly leave the other drives in and set the 4th drive as the boot priority so that grub detects your Windows drives and sets up the boot loader properly allowing you to choose on start up.
But honestly, I would just save yourself the headache if you aren't used to Linux or Dual Booting then it's not for you right now. Remember SteamOS is still very much Alpha / Beta stage and things are going to continue to change until it's final release. Do expect bugs, do expect troubleshooting, do expect to have headaches if you've not used Linux before.
I would much rather suggest to install Ubuntu or Mint instead as they'll be much easier to dive into requiring little to no real troubleshooting while still providing you with Steam. Remember, SteamOS is just Debian with Steam pre-installed and Steams own repositories. Nothing too special.