-If the hdd's were formatted and had windows installed as a raid-0 pair. Trying to run one drive without raid-0 or the two drives as non raid drives will mean they wont be formatted to run that way. The drives would be unreadable by themselves.
-The drives have to run together in raid-0 if they were formatted that way. Each drive will have half of the data since they were running as one drive.
-You will need to format both drives if you want to run them as separate drives. (quick format should be fine)
-A bit of speed can be gained if you partition the os drive in half, the first half of a hdd is faster then the second inside part of the platters. The second partition could be used as a slower media/music drive.
-You could install some games, programs, steam program folder to the second hdd. Since it is a separate hdd
Windows sometimes needs to read both game/program files and os files at about the same time.
-I run my systems this way. It is much easier than running raid-0. Also you don't have to worry about having matched drives and more reliable than raid-0. It is also much easier to upgrade a hdd later on by simply downloading the software from the new drive's site, then it copies the old drive to the new one.
-It works out faster than a single drive, but not as fast as raid-0, a kinda compromise between the two.