Hi, thanks for the reply. My Bios can't be changed, however, I learned that AHCI is set by default, so thankfully, this isn't the issue. When I received the error message "waiting for root device" was when I installed the HDD again internally.
Here's the issues I have:
1) If I attempt to install OS X El Capitan via bootable USB, the installer won't detect internal HDD. (It will only detect it if I remove it and plug it in as an external HDD via USB)
2) Once I managed to install OS X and then install the HDD back to its original place, it will give the error message "still waiting for root device".
I learned also on other forums it might be due to SSDT patching, but I'm not very technical, so I'm not sure how to do that.
Please let me know what else can be done. Thanks in advance!