Windows doesn’t really care at all, for reasons that we often overlook.
We need to start by clarifying something: the initial steps of booting your computer have nothing – or at least very little – to do with Windows.
Let’s have a look at booting, who does what at boot time, and why you probably can boot from an SD card if you really want to.
What it means to boot
To “boot” something is a term derived from “bootstrapping”, which in turn came from the phrase “to pull one’s self up by one’s bootstraps”.
In the realm of computing, to boot something means the process of starting up a computer or similar device from some inactive, initial state, to a working, functional state. It’s mostly about loading the operating system software, such as Windows, but can involve not only that but any software configured to run when a computer starts.
Note that I said “load the operating system software”. That’s our clue as to why Windows doesn’t so much care about what you boot from.
Boot time chicken and egg
Windows doesn’t control the initial state of the boot process because Windows isn’t running yet. The boot process loads Windows1, but booting starts before Windows is around.
In fact, it doesn’t have to be Windows at all. Your machine could boot into Linux, or even some other operating system entirely. Not only is Windows not involved at that point, it may be nowhere near the machine.
BIOS and UEFI
The computer’s BIOS is built in to the hardware. Aside from some housekeeping when it starts, it’s there and running the instant you turn on your computer.
A BIOS has several different jobs, one of which is to load the operating system.
In other words, it’s the BIOS that does the booting.
It’s your computer’s BIOS that determines what devices you can boot from.
If your computer has a built-in SD card reader, and your computer’s BIOS supports booting from that reader, then you’re good. It should, in theory, work.
In reality, most computers don’t have a built-in SD card reader. Most computers do have USB sockets, though. As a result, it simply makes more sense to make the USB interface the next most logical boot device after the hard disk itself, or any optical drive.
That being said, most SD card readers are, themselves, external devices you connect to your computer via a USB interface. Therefore, you should be able to create a bootable SD card if you treat that external SD card reader as if it were a USB thumbdrive.
But if you need to have an external card reader and a SD card, it’s probably just easier to use a plain old USB thumbdrive to begin with.
Where many people get hung up, particularly since the introduction of UEFI, is boot order.
When you turn on your computer, the BIOS essentially asks itself a series of questions to figure out what to boot from:
- Is there a hard disk? Is there an operating system2 on the hard disk?
- Yes? Great, let’s boot that.
- Is there a CD or DVD drive? Is there a CD or DVD inserted? Does it have a bootable operating system on it?
- Yes? Great, let’s boot that.
- Is there a USB drive? Does it have a bootable operating system on it?
- Yes? Great, let’s boot that.
- I give up. No bootable devices.
As you can see, if there’s a bootable operating system on the hard disk, the USB drive will never be used for booting.
Most BIOS’s let you change the order it checks. A very common alternate order is to check the CD/DVD drive first, then USB, and then lastly the hard drive. As long as both CD/DVD drive and USB drives don’t have anything bootable inserted3, the machine boots from the hard drive; otherwise, the inserted disk or thumbdrive is used.
How you change the boot order depends on the BIOS in your machine, and naturally, different machines have different BIOSes. You’ll need to check the documentation that came with your machine.
UEFI can make things more complicated, because it provides a mechanism to prevent changing the boot order at all, or to prevent booting from anything other than the hard disk.
And this is also where Windows can get involved in the boot process, kind of.
UEFI, booting and Windows
With the advent of UEFI, many of the choices we might make at boot time are now made in Windows itself instead. For example, to reboot into a recovery mode, to repair Windows, or even to change many different start-up options, Windows 8 and 10 use the Settings app to make those choices. Then, under the control of the UEFI, the machine reboots and whatever was selected happens.
To me it feels somewhat like putting the cart before the horse. You have to boot into Windows, (usually proving you’re the administrator – an added security measure), in order to run the settings app to make some of the choices affecting how the machine boots the next time. But it’s still a reboot. It’s the UEFI that determines what happens, though at the instructions of a previously running copy of Windows.
One of the more frustrating aspects of UEFI is that it can disallow booting from external media at all. This is a security measure that prevents people from walking up to your computer, inserting a disc, and forcing a reboot to bypass all of your other security. Unfortunately, it’s a hassle if you have a legitimate need to boot from something other than the hard disk.
Once again, this also all depends on your specific computer and exactly how your computer manufacturer configured the UEFI used in your machine.
If it looks like a USB drive
One of the nice aspects of the USB interface is that it more or less hides the technology being used from the computer. If your USB SD card reader looks like a USB flash drive, you should be able to use it as a boot device. Similarly, one of my recommendations for booting from DVDs and CDs for machines that don’t have optical drives is to get an external USB DVD reader. Most look like any other USB device at boot time, and if the BIOS supports booting from USB, they can be used for exactly that purpose.
So, sure, you can boot from an SD card if you like. You probably just need to treat it like a USB flash drive.
I just don’t see any huge advantage to doing so.