Open Disk Management (Windows Key + X for menu).This version allows us to convert MBR booting Windows 10 into an UEFI booting version quickly.
In case you haven't already given in to Windows Update's nags, you should first update to the Creators Update (Version 1703, Build 3) or later. You can now put away the Apple computer and switch your attention over to the Thinkpad T420.
Sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia -volume /Volumes/USB -applicationpath /Applications/Install\ macOS\ High\ Sierra.app -nointeraction
Since this is the only step we will need a Macintosh for, we will go ahead and do this first. I went with an inexpensive HP M700 120GB SSD and this ultrabay caddy. A secondary hard drive: you can either get a mSATA or a laptop HDD but if you go with the latter, you will need an ultrabay caddy to hold one of the hard drives.I recommend the Dell DW 1510 AGN Wi-Fi card. A macOS-compatible Wi-Fi card: the one that came with the T420 is not compatible with the OS.Access to a Macintosh and a 8GB or larger USB drive you will need them to download macOS High Sierra (10.13).Speaking of starting, there are a few more things you need before we begin.
I took advantage of the free upgrade to Windows 10 and have not found any issues driver-wise so the guide will start from this assumption. Like most Thinkpads, my T420 came out of the box running Windows Windows 7 to be specific. This is a guide for turning a Lenovo Thinkpad T420 into an even more powerful multi-tool running both macOS and Windows 10.
While Apple laptops are my gold standard for laptop design and usability, there is a special place in my heart for old-school Thinkpads that are built like tanks and ready to go to work wherever they are, especially with their full array of ports, removable components, and roll cages.