Yeah, you can go about this multiple ways. If you take your old harddrive out while testing the new OS its 100% safe, or you can keep both and load a bootloader that will allow you to choose between them both at startup (although sometimes this can go wrong). Other options include emulating another PC in VMWare or Virtual PC on windows. Personally I'd just swap the drives around if I was really concerned about data on my existing harddrives, but the bootloader option can be convenient (just my
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