Continuing the discussion from Pre-installation problems: Thinkpad X220 won't boot from USB:
I am now able to boot, but as this machine supports both UEFI and Legacy mode, I am wondering what option I shall pick and why. What are the differences, advantages and disadvantages? I am intending to run UM on a Thinkpad X220 with an SSD. Windows 10 is pre-installed and I intend to erase it, so no dual boot.
I would go with Legacy mode. It “just works”.
UEFI is more rocket science. And you never know what you will get. UEFI has a different implementation on every computer, while Legacy should work the same on any computer.
UEFI is a big insecure pile of crap (just my opinion). I have heard that UEFI is the size of the Linux kernel or bigger. For what? So you can have a mouse cursor maybe…
I might add that Legacy mode is still UEFI, not the BIOS of old. But Legacy mode is still better I think.
So I just disable UEFI boot mode in the BIOS and I am good to go? I have also heard that the type of harddrive (in my case SSD) and the file system needs to be considered. What do you think? @wolfman
Yes, disable UEFI and make sure the USB drive boots in Legacy mode. Ext4 should be good for SSDs.
With my computer changing the boot order requires a UEFI password. Once the password is set, I can Disable UEFI Secure Boot (But it works fine with the secure boot on), and change the boot order to boot from DVD Drive and USB Devices first.
For some reason the settings refused to save if i didn’t create a UEFI password.
After installation I go back into UEFI and set the OS Boot Order to boot from Ubuntu first rather than windows first.
See the UEFI section here too!:
Edit: That’s secure boot, a feature of UEFI. 
Discussion below.
UEFI is still relatively new, so it’s up to you (and your computer) whether you’d like to stick with what’s been around the longest, or utilise new technology that will eventually be a successor to BIOS and it’s limitations.
BIOS is old (from the 1980s I think) and needed a few fixes. But it needn't be replaced by an entire OS, that an OEM will never be able to keep secure. It's the ideal environment for backdoors considering the sheer volume of code and proprietary nature.
UEFI also has scary features like injecting code during boot-up (a Windows "feature") that Lenovo used to inject adware/spyware into clean installs of Windows.
Intel hijacked the UEFI spec 10-15 years ago and made it into the mess it is today.
Like a bad version of UEFI I injected my comments into the quote below:
UEFI Advantages
Supports booting from > 2 TB disks.
Yes, but UEFI requires its own boot partition.
Supports booting from GPT partitioned disks.
From How to Geek site: It’s called GUID Partition Table because every partition on your drive has a “globally unique identifier,” or GUID — a random string so long that every GPT partition on earth likely has its own unique identifier.
This is another feature that just doesn't sit right with me.
Overcomes limitations by MBR, like a maximum of 4 primary partitions.
You only need primary partitions for Windows.
GPT is saves it's partition table to both ends of the disk in case one got corrupt.
If MBR breaks it's easy to reinstall GRUB.
UEFI could be faster...
No 
Signed OS and firmware can mean fast boot times (like Windows 10, but haven't seen this under Linux)
Windows 10 doesn't boot faster, it uses hibernation and calls it fast boot.
Arguably secure to prevent anything from overriding the boot loader.Where's the freedom of Linux in that?
Secure boot is about protecting Microsoft, not the user. It should be an integrity check of the OS like Chrome OS does it. It should protect (verify integrity of Windows) not hijack the computer.
UEFI Disadvantages
Can be fiddly to setup.
Everything else (CDs, USBs) for booting must have EFI support.
Agree.
BIOS Advantages
Just works™
Being around the longest, has more support.
Agree.
BIOS Disadvantages
It's old technology.
Agree.
UEFI is still relatively new, so it's up to you (and your computer) whether you'd like to stick with what's been around the longest, or utilise new technology that will eventually be a successor to BIOS and it's limitations.
What I don't like is that they are trying to sell us a proprietary OS with low level control of our computer. They justify this by saying that BIOS is old. Yes it's old, let's update it instead of creating a monster that goes against everything that Linux and open source stands for.
I've heard of these bad things with UEFI and Windows, I had overlooked them when thinking of advantages and disadvantages, so thanks @mrtribute for pointing them out.
No surprise with Windows/Microsoft following an Apple-like approach by unifying the hardware and software together. In principal, this can be used for good (like Chrome OS 
) but also for bad (like Lenovo 
).
The GUID Partition Table is definitely superior as it's more redundant. With each partition assigned an ID, it's easy for the computer to identify.
GPT 
"This partition: 1234-5678-9012-3456"
MBR 
"It's partition 2 on drive 1, yes... no, it's supposed to be EXT4!"
I've heard GPT disks can be booted in BIOS mode, but it's very hacky and not worth the messing around.
If MBR breaks it's easy to reinstall GRUB.
I meant the actual partition table, not the bootloader (which a bit of code lives in the MBR too). If the start of the disk got corrupted, it would need recovery tools to scan the disk where the partitions are.... If it was a GPT disk, a copy is stored at the end, making it a bit redundant.
I've also read that it stores cyclic redundancy check (CRC) values across the drive, so if the disk is beginning to corrupt, GPT will know right away.
You only need primary partitions for Windows.
You can use primary partitions for Linux too. Like one for /, /boot, /home and dual boot situations. Extended/logical partitions was like a trick to overcome that limit. Windows can't boot from them, but Linux can.
What I don't like is that they are trying to sell us a proprietary OS with low level control of our computer.
BIOS is arguably proprietary too, as many motherboard manufacturers haven't released the source code for their BIOS. 
I see UEFI / GPT as modern... it's just a pity the large corporations behind it may use it for evil intent.
It's an interesting conversation since we're talking about UEFI vs. BIOS/Legacy, which also presents a choice (and more advantages/disadvantages) to picking how your disk is formatted. 
These are great features for the data center. I have been using computers for 20 years and I have never had a HDD fail. Of course it can happen, but do consumers have the tools to fix these problems if they arise or maybe you mean that GPT is sort of self-healing. When you make stuff more complicated professionals can take advantage of it, but consumer hardware should be as simple as possible I think.
Another feature for the data center. I am not saying these features are bad. It's like ZFS - great for the data center, but overkill for a laptop.
I am not so sure I want hardware to perform a cyclic redundancy check. I stopped using Windows to get away from "automatic maintenance" and background processes.
I want consumer hardware to be simple and disposable. If you care about your data (we all do) make back-ups (local or to the cloud). I don't want/need data center features on my PC.
The biggest problem with UEFI/GPT is that it's hard to install Linux. In the days of BIOS most people could do it. Now even technical people scratch their heads and read online tutorials to get it right.
Having a UEFI spec without a single consistent implementation is a big part of the problem. Just because you managed to install Linux on one UEFI computer doesn't mean you will be able to do it on another UEFI computer.
Those were the biggest strengths of BIOS. Simplicity and consistent implementation.
Yes, but it's small. Small is good. Simple is good. Some people like advanced things. I like simple things. At the end of the day it's about personal preference.
I think this thread highlights pros and cons of both UEFI/GPT and BIOS/MBR.
Thank you both for your in-depth discussion of these issues. Even though I am mainly concerned with solving the technical aspects for now, I feel that this discussion is very valuable in clarifying to what extent firmware and perhaps hardware is “open”. Having control over your system may begin for many with the choice of OS and software, but firmware and hardware are largely beyond our control unless there is more consumer awareness to influence corporate decision-making of manufacturers.
I have now set Startup > UEFI/Legacy Boot to Legacy Only. Booting from USB is no problem then.
After setting Config > USB > USB UEFI BIOS Support to Disabled, I cannot boot from USB anymore.
How can I then proceed with installing UM in Legacy mode? I am basically confused about how these two options are related to one another. Are both settings, that is, Legacy Only and disabled USB UEFI BIOS Support necessary to install in Legacy mode? If necessary, I can provide screenshots.
What happens if you enable it?
I don't think both are necessary. This is why I don't like UEFI. Things become difficult.
Since you just want Ubuntu MATE and not Ubuntu MATE plus Windows it doesn't really matter if you install in UEFI mode or Legacy mode.
I install in Legacy mode because I always install multiboot setups. I find it to much work to set up multiboot with UEFI.
My advice is: It doesn't matter if you install in UEFI or Legacy mode. Just let the installer do its thing. I am pretty sure the Ubuntu installer is able to set up a fully working Ubuntu MATE installation as a single OS.
UEFI gets difficult in multiboot environments, but shouldn't pose a problem with a single OS.
If I have USB UEFI BIOS Support enabled, I can boot from USB. On my X220 Windows 10 is pre-installed and when I switch Startup > UEFI/Legacy Boot to Legacy Only, Windows 10 is still booting. Is this normal? I have not started with the UM installation. Still testing the boot options.
If Windows is installed in UEFI mode then it shouldn't boot in Legacy mode. But I am not completely surprised. I have heard things like that before.
It's good that Windows works. I would keep Windows and shrink the Windows partition. But the Ubuntu MATE installation might become a little trickier if you decide to keep Windows.
If you install in Legacy mode just install GRUB to MBR (sda) and it should find both Windows and Ubuntu MATE.
I got this X220 second-hand. Its current Windows 10 Pro installation was probably updated from Windows 7 or 8. I will set UEFI/Legacy Boot to Legacy Only and have USB BIOS Support enabled, otherwise I cannot boot from USB on this machine. Not sure if I would then be installing in UEFI or Legacy mode?
I think you can look at the initial boot screen of Ubuntu MATE to determine if it is booting in UEFI or Legacy mode. If you see a "themed" (grey, transparent) screen with selections:
Try Ubuntu MATE without installing
Install Ubuntu MATE
Check memory
etc
you are booting in Legacy mode. If you see the same screen without theming (black background, white text) you are booting in UEFI mode.
I will try to find a link that shows the difference.
Found it. This site shows the difference in Ubuntu.
https://help.ubuntu.com/community/UEFI
With Legacy the boot screen is themed. With UEFI the boot screen has no theming. I think this applies to Ubuntu MATE as well, but I am not 100 % sure.
The UM installation was successful. I decided to experiment with it and install UM 16.04 alongside Windows 10 to see what happens. Thank you for pointing me to the screenshots to determine whether it boots with Legacy or UEFI. This was extremely helpful. In my case, both Windows 10 and Ubuntu MATE are booting in Legacy Only without any problems. When I switch to UEFI Only neither of them is booting.
UEFI ensures your Windows is locked to that computer only.
Not all Motherboard BIOS require that. My ASUS doesn’t.