How do I know if I should do UEFI mode or Legacy mode when making the USB?
Do you already have another OS installed? Since this is a fresh install the answer is "No". Do you plan to install another OS besides OSX? If yes, do you intend to install the OSes on separate disk drives or on the same disk drive? Will you be using bootable disks greater than 2TB? (If so, select UEFI.)
When you install through UEFI the boot loader will reside on the EFI partition and it will be the only boot loader for all disks. So if you decide to go with two separate disks, say one for OSX and one for Windows, then both OSes will boot from the boot loader on the very first disk, usually SATA0, if that was where you connected the SSD or HDD. (In the old days one would connect the CD/DVD drive on channel 0 (SATA0 or IDE0). I installed my CD/DVD burner on SATA5 (the last channel) and the BIOS can still boot from it. Some motherboards will state that bootable disk drives should be connected to either SAT0 or SATA1.) If that disk fails then no disks can boot.
With Legacy boot each drive can have its own boot loader and then you would have to chose which to boot by hitting the boot key, say, F11.
If all the disks are written in UEFI then Clover can be used to choose which OS to boot. If Clover is in UEFI and the other disks are in Legacy then the Legacy disks cannot be booted through Clover, just as no Legacy disks can be booted even though CLover will use an EFI partition (if your mobo allows both UEFI and Legacy booting at the same time.)
With UEFI enabled you will probably need to disable CSM in the BIOS. If CSM is enabled in the BIOS then the OS may be installed in Legacy even though you told it to load in UEFI (usually affects Windows, maybe some Linux distros. In the case of Linux Mint Ubuntu will take over the EFI boot loader and call it "Ubuntu". You will then go through gyrations to make Windows, for example, work correctly.)
If you decide to install in UEFI mode then make sure that you create UniBeast with UEFI boot loader and when selecting it as your boot device in the BIOS make sure that you select "UEFI-USB" otherwise it will probably fail (even after disabling the Serial Port, disabling Safe Boot (which is for Windows and Linux), disabling VT-d, etc.
Once you install OSX through UniBeast run MultiBeast, without first rebooting, select your options and make absolutely sure that the disk drive is selected as the place for MultiBeast to save its files. If you have an nVidia card that also needs to have WebDrivers installed you may opt to install them after running MultiBeast or, if you have internet access, select to download the WebDrivers. Just be aware that the latest WebDriver may not work correctly. It is probably safer to shutdown the PC after running MultiBeast, remove the UniBeast USB stick, then power up the PC. Hopefully it will boot into OSX okay, albeit in low res. You would then download and install the nVidia WebDriver.
UEFI should allow faster startup, shutdown, sleep and wake up times. Legacy will take longer. UEFI should be impervious to root kits, (Legacy can address the lower 1MB of RAM) but... (Safe Boot Mode should be turned off to allow booting other OSes, like OSX and many Linux distros.)
Make no mistake, UEFI has a deep learning curve. You'll probably initially learn to hate it. After using it awhile you will probably like its many features, especially the Clover UEFI Shell. "Shell" = mini OS, like DOS, BASH, Windows Recovery, etc. For example, if you install in UEFI mode with CSM enabled you may find a BIOS entry called "MAC OSX" instead of "El Capitan". You will then need to go into the Clover UEFI Shell to remove that entry. You may also find extraneous devices listed, which I assume get written every time it goes into Sleep or Hibernation Mode and wakes up. Then you will need to go into the UEFI Shell and remove them. (Separate thread elsewhere.) Tjose extraneous entries probably won't be written in Legacy Mode, or if you never enabled the BIOS C-States 6 and 7 and you always shutdown your PC instead of allowing it to go into sleep. I tend to think that Sleep and Wake are more for Laptops than desktops and Windows Hybrid Shutdown doesn't help.
I usually suggest that you make two UniBeast sticks, one for UEFI and one for Legacy. Just in case...
And once you do install it and everything works fine, do NOT overwrite that stick. Treat it like GOLD. Make sure that it is protected (nice little case that you know where it is at all times. Don't leave it laying around and then drop a can of Coke on it.)
