|Of course, the hard drive you boot from must have an operating system installed on it in order for you to be actually able to boot from it. |
"Were both instances of Windows installed while the hard drives were installed in this computer?"
I'm assuming each hard drive boots fine when connected by itself, and you don't have a dual boot situation.
Bioses have boot order defaults they adhere to. Those defaults can vary. Sometimes the defaults aren't suitable for your situation.
In your case for your bios, if there is a PATA hard drive, or a PATA and SATA hard drive, connected, the bios defaults to try to boot from a PATA hard drive first.
When you disconnect the PATA' drive, if a SATA drive is connected, then the bios finds no PATA hard drive and then tries to boot a SATA drive first.
Some bioses do the opposite by default - if a SATA and PATA hard drive are connected, it tries to boot a SATA drive first.
If the bios isn't booting the drive you want to boot, it's easy to change that.
When both drives data cables are connected, go into the bios Setup and find the Boot Order settings or similar.
- if you have more than one hard drive connected, the hard drives are selectable in the boot order settings - e.g. HDD0, HDD1, etc., or the model numbers are there
- or - more commonly - the boot order has only one "hard drive" or similar listed, and, if you have more than one hard drive connected, there is a list of hard drives there near the boot order settings, usually listed by their model number.
In either case, make the hard drive that is the drive you want to boot from, in this case the SATA model, the first drive listed.
Save bios settings. Reboot.
If you have more than one PATA or more than one SATA hard drive connected, the bios defaults to try booting what it sees as the first drive on the drive controller. If that isn't the drive you want to boot from, you would need to change first drive booted from settings in the bios for that too.
Note that if you disconnect a data cable to a drive, then boot the computer before reconnecting it, or change the number of drives you have connected or where drives are connected, the bios may set itself back to defaults regarding which drive is booted from.
Some older SATA controllers on mboards have some SATA connectors that a drive connected to it can be booted from (the drive is seen as master) , and some SATA connectors that a drive connected to it can NOT be booted from (the drive is seen as slave). Sometimes the two types of SATA connectors are different colors, sometimes they aren't - see the mboard's manual or the User's or Owner's manual for a brand name system.