Although this question gets asked a lot in different forms, and this particular one has already been answered be several people here, I want this answer to be used as a reference for future people coming with this question.
I say formatting the partition and reinstalling Windows is not
the safest option. You should delete the entire partition table from the disk using a Linux Live DVD, (the one Arch Linux provides on it's website is excellent for that and you can accomplish this intuitively with the
cfdisk utility), and create multiple different-sized partitions on that disk to thwart any BIOS malware (rootkits) that guesses partition sizes from installing programs into it, format them all with Linux filesystems like ext4 - which can't be read by Windows, implies you have to install Linux - if you want to keep them all separate, or you can combine all these partitions into one filesystem by making them all a Linux LVM partition.
Some manufactuers put rootkits in the BIOSes of Windows computers they sell that automatically installs their junkware after a Windows reinstall. These depend on being able to read an NTFS or FAT32, or basically any other type Windows ever supported officially, partition. Some of these rootkits are old can only read legacy-format MBR partition tables so it's better to create the new one on your disk to use the new and modern GPT table as well. It can also be read by Windows 10 and that's why I singled out old rootkits.
At least one major vendor (Lenovo) did this in the past for some of their models, but not on Thinkpads, only on home models like Yoga and the like.
The basic idea for doing all this is to
prevent Windows rootkits on the BIOS from working even though they cannot be uninstalled.
Also, using Windows as your OS for cold storage introduces the additional problem of 1) missing out on Windows security updates, and 2) Electrum making a version that indirectly no longer supporting your version (build number in Windows 10's case), because its dependencies dropped support for it. The only way to resolve that would be by connecting the cold storage online to use Windows Update, a big no-no, or by formatting some removable media with a new Windows version and reinstall that.
On Linux it's actually possible to download new versions of all installed packages onto removable media and connect it to the cold storage and update the system from that. This solves 1) and 2) at the same time.
Ok, I don't want to make a new thread since this one is related anyway, but what other Linux distros are good for air gap installation with Electrum?
In Arch Linux it isn't even easy to bring up networking after you do a fresh install. I did install it a few days ago and it was only when I had dhcpcd, systemd-networkd and systemd-resolved all running I was able to get network connectivity from an Ethernet line. If you refrain from installing any network-related packages via its live DVD - because nothing is installed by default and in Arch all the packages must be installed by hand -, it is perfect for cold-storage systems since there's no way for malware installed to connect to the internet even if it wanted to.
I hear Gentoo is similar to Arch except you have to compile your own kernel too, never tried that distro so can't comment about it further.
Otherwise most other distributions automatically set up networking and are equal in that regard.
It literally doesn't matter which distro you use as long as its a reputable one and not something like hannah monatana linux.
Well actually it matters only if you don't want to reinstall newer distro versions because you'd want to use a rolling-release distribution that has perpetual support, or at least a fixed release distribution like Ubuntu which has a special "OS upgrade" command for when your release becomes EOL. This is not a real issue, just an inconvenience.
