swiftninja, you repeatedly posted the same picture (BIOS screen shot). Can you edit your post?
Now to the points you answered:
AHCI (or whatever) - good, it shouldn't be IDE.
"Windows boots in both EFI and legacy": I suspect it always boots in UEFI mode on your system, since disk 1 is identified by the BIOS as UEFI drive (see below).
1. You have selected UEFI boot in the BIOS options.
I haven't selected that of my own free will. The disk drive always appears as UEFI
As you wrote in your reply, you updated the BIOS and you or the update probably selected "Legacy ROM". Keep it at that for the moment.
2. You are trying to boot from disk no. 1 in UEFI mode, which should be your first hard drive in your PC.
Without the USB plugged in the boot order is DVD Rw>DVD drvie> 1TB Seagate>old Samsung HDD
OK, I suspect that Windows is installed on the 1TB Seagate drive that is identified by the BIOS as "uefi"? Right? If I remember correctly, when you install Windows 7 from DVD on an UEFI capable PC it should default to UEFI installation. In any case, your BIOS identifies the disk as UEFI disk, which means it has a GPT partition table and a fat32 EFI (or efi) partition which holds a filename.efi bootloader file. The file structure in this partition is something like /EFI/efi/microsoft/ with the .efi file in the microsoft folder. Anyway, you can check that later once you successfully boot Linux Mint. An UEFI capable motherboard is able to read GPT partition table and the EFI partition and load the OS using the .efi file specific for the OS.
At least now you are trying to boot in legacy mode (as opposed to the UEFI mode you used before).
I would like to see the other screen shots, especially for the boot sequence, since it might give some clues to what's wrong.
Here are some other tips that may help:
1. Disconnect all DVD drives. There may be a problem with one or both of them, or with the way they are connected (the ports you use on the motherboard). You can always hook them up later.
2. Check the SATA ports you used to connect the Seagate and the "old" hard drive. Here are the specs copied from the Gigabyte website:
2 x SATA 6Gb/s connectors (SATA3 0/1) supporting up to 2 SATA 6Gb/s devices
3 x SATA 3Gb/s connectors (SATA2 2/3/4) supporting up to 3 SATA 3Gb/s devices
1 x mSATA connector
Check that the Seagate drive is connected to the SATA3 0 (the first 6Gb/s) port, the other hard drive to SATA3 1 (the second 6Gb/s) port, or to the SATA2 2 port (the 3Gb/s port). Connect the DVDs to SATA2 3 and 4 later on.
3. If I'm not mistaken, your CPU has it's own GPU? At some point during boot it may switch to your CPU internal GPU, or vice versa if you connected your monitor to the internal GPU. Have a look at this post http://forums.linuxmint.com/viewtopic.php?f=42&t=109298
, perhaps it provides some clues. Look for the "nomodeset" boot option.
Check your BIOS to see which GPU is selected. I'm totally unfamiliar with HDMI, but you may want to try to connect your screen using a DVI connector.
But first of all let us have the screen shots! They may give a much better clue.