I installed a SATA 1.5TB hard drive into my Dell Dimension 8300. The computer recognized it and I have been able to write to it. The HD I bought the computer with is still in my PC and contains the OS. When I turned off my computer, it tried to boot from the new SATA drive, not the original HD. The only way for me to boot is to hit F12 when starting, then selecting my Primary Hard Drive. Looking at the sequence in the Boot Menu, it looks like it wants to boot from my new SATA, then my Primary. Can I change this to look at my Primary first. I have tried to change the BIOS, with no luck.
You should be able to change the boot order. Try to high light the drive you want to boot from and then use the PageUp or UpArrow keys to move the drive up in the boot sequence. Each BIOS is different so I can't tell you the exact key sequence.