Help! BIOS wont load windows. Need to UEFI boot. (old comput

Asus G73sw Laptop that I have used every day for the last 2 years.

So, I got the message: "Reboot and Select Proper Boot Device, press any key" so I tried another working HDD that has windows 7 on it; same message. I took apart the computer and checked the connections and cables. One time, during the constant restarts with other HDDs, I got the message "A Disk Read Error Occured." When I entered the BIOS menu, It recognized the HDD. Also, on the Easy Flash, I could see that HDD and its files.

Finally was able to load windows with UEFI mode. I am unaware what this is, but I assume it will work for now?? I have made no updates to windows 7, installed no drivers or programs recently, and am unsure why BIOS won't load any of my HDDs to windows, when BIOS clearly sees it.

Can anyone help me with this issue?

Thanks