Are Systems 10.5.8 and 6.03 compatible if on separate drives on a Mac Pro?

Question

Question:  I am running System 10.5.8 on  a 2008 Mac Pro in order to preserve my ability to use certain legacy applications such as Quicken 2003 and not lose years of financial data.  There is no reliable financial application currently available to imput Quicken 2003 QIF files to but I need to upgrade my System 10.5.8 to Systen 10.6.3 in order to run TurboTax 2012 from a DVD.  Because the 2008 Mac Pro has 4 bays my thought was to add another drive and install System 10.6.3 plus 10.6 updates to 10.6.8 on the added drive.  This action would preserve the ability to run legacy software for awhile on System 10.5.8 and permit the switching between the two drives and systems for flexibility.  My concern is if this action is undertaken possible firmware changes in System 10.6 would destroy the ability to use System 10.5.8 on my current drive and destroy my ability to access my years of financial records on my drive that are contained in Quicken 2003 and past TurboTax  years.
    The major issue is whether Systems 10.5.8 and 10.6.3(plus updates) can co-exist on a 2008 Mac Pro if installed on separate drives?

Answer 1

To Hatter:
Thank you for the comments, they were helpful.  I agree that using a separate drive for System 10.6.8 would be wasteful of capacity but am very concerned that if hidden firmware changes in System 10.6.8 affected the ability to preserve the usage of System 10.5.8 on my current drives in bays 1 and 2 there would be no way back.  I accept the the wastage by believing it may be more conservative and safer than partioning my drives 1 and 2.  I back up my drives 1 and 2 to external firewire OWC drives using Super Duper but restoring from the backup drives might not be possible if hidden firmware changes in System 10.6.8 modified the Mac Pro.  Again, there would be no way back I fear!
    Keeping System 10.5.8 alive keeps my historical financial records alive until some software house develops an application that functions reliably and can import and preserve historical Quicken Quicken  2003 QIF files.  It also preserve the ability to reach back for tax records given that TurboTax now has a minimum system requirement of System 10.6.0 starting with 2012.
    Also, I understand your point about having the system and software on separate drives but I have a need to maintain data files on separate drives due to Quicken 2003 software deficiencies—it won't handle two sets of books on one drive without chaos.  Separating System 10.5.8 and the two data collections would require three drives and impact the backup process that I diligently follow.
  But your thoughtful comments have helped me with my deliberations.