I'm trying to set up a multiboot on a laptop, but that's irrelevant to the problem I'm having. I have partitioned my drive to have a boot (bios_grub), swap, and several ext4 partitions for my multiboot. As of now, I have only installed Mint on the third partition (following the boot and swap partitions).
When looking at the boot menu in BIOS I see nothing, unless I have my YUMI thumbdrive plugged in, in which case I can see and use only it. When booting through YUMI and selecting to boot through First HD, it puts me in the GRUB menu and I can select my Mint installation and all is well. The BIOS is definitely not UEFI, though if I recall correctly I can do some sort of override that didn't work without the thumbdrive plugged in.
Also, when I look inside the SATA Configuration menu under the Advanced tab, I can see both the HDD and SSD.
My current suspicion is that it has something to do with the secondary SSD that, if memory serves, used to boot windows 8, because from everything else I've seen it was a pretty simple fix involving resetting BIOS settings to default, disabling Secure Boot, and enabling CSM, all of which I've done. The only thing I haven't tried so far is physically resetting the CMOS settings, which I might just do after posting this because I am really at my wit's end with this, not that there was much wit to begin with. I might also try to unplug the SSD and see if that does anything while I'm inside anyway.
The computer in question (I'm fairly certain) is the ASUS TP3000LA, I believe the motherboard has the same name.