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Why Can’t I Boot from an SD Card?

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I have noticed many times in publications that we can use a USB flash drive to boot from under certain situations. But what about SD cards? Can we also use SD cards in the same way as we can use USB drives? I have noticed on my computers that the computer seems not to differentiate between the different types of drives. Does Windows really care if you use a USB drive versed to an SD drive for doing these sorts of things like booting from?

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.

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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

What controls the booting process is the computer’s BIOS, or UEFI, the more powerful and secure BIOS-replacement on newer-model computers. I’ll simply refer to both as BIOS.

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.

Boot devices

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.

Boot order

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.

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Footnotes & references

1: More correctly, it loads a small program that loads a somewhat larger program, that then loads another larger program which is typically Windows, which then finishes up the whole process. But that’s beyond the scope of what we care about here.

2: Technically, “is there a boot record?” That boot record may, itself, contain a small program to make further choices about what to do. Typically this is how dual-boot or multi-boot systems are set up.

3: Sadly, some BIOSes are bad at this. Sometimes any connected USB drive, whether it has an operating system on it or not, will be enough to cause the BIOS to try to boot from it … and then fail.

10 comments on “Why Can’t I Boot from an SD Card?”

  1. To complicate things further, some BIOS allow you to change the enumeration order of the connected hard drives. Recently, it happened to me that that order got accidentally changed, making it look as if the OS got corrupted or a key file got deleted.
    Reordering the drives in the correct order saved me that time.

  2. I do have a built-in multi-card reader, and because of some difficulties I had with my built-in USB ports, I opened up the machine and while I was checking connections, noticed that the multi-card reader was connected at the same place on the motherboard as the USB ports, and I believe the exact connection is also labeled USB.

    Does this mean that my built-in multi-card reader is really just another USB device and an SD card would be treated the same as a USB flash drive?

    This is an interesting question, even though like Leo, I’m not sure why I would use it. I haven’t seen any SD cards big enough to turn into a bootable version of Windows, and even if there are, they seem to be a lot more expensive than comparable USB flash drives.

    • It *may* be. I know that some manufacturers do exactly that – implement the internal devices using USB technology. But it’s certainly not required that they do so.

  3. I find one advantage of booting from an SD card, probably not a major advantage, but it’s something I’ve thought about for a while. For example, if someone is worried about malware, and they prefer doing their banking using Linux live, they can keep an SD card plugged into their laptop and boot from that when they want to do their banking. I’ve never done that, and the person I know who does uses a DVD, but with many laptops not coming with optical drives now, booting from a permanently installed SB card might be nice for that.

  4. I have a suggestion–most of the SD cards sold today are of the “Micro” variety, and the make “hollow” thumb drive carriers or adapters to hold them. (Cheap–eBay has some for $.77 from China.)

    So you could literally plug your SD Micro card into the adapter, plug it into a USB slot, and you’re off to the races! The PC thinks it’s a USB drive.

    Hope this helps someone,

    –Al–

  5. The clear advantage to using an SD card is that it doesn’t stick out of the machine and won’t break off. Card readers are common on most laptops – and at least one manufacturer is making a laptop which comes with basically no storage other than 32 GB for the Win10 OS and 3gb of RAM. Memory and storage expansion is through SD card and M.2 SSD. It retailed for $169 CAD on sale at my local shop. Tempting! Except for the Celeron processor, of course.

    EUFI can generally be configured to boot an OS from any drive, but the EUFI partition itself needs to be on a bootable device.

    If desired, the card could also function as a security key, but it doesn’t seem sensible really.

    That said, not all card readers are created equal. In my 2014 Lenovo G400s, for instance, the card reader is connected to the USB 2.0 bus, which isn’t fast enough to support ReadyBoost, so it’s unlikely to provide any performance improvement over the HDD. A USB 3.0 interface would show a noticeable improvement though. In both cases however, the SD card itself will probably wear out faster than an SSD if used for things like the page/swap file – it generally doesn’t have the advanced internal management features or “spare” capacity of a fully-fledged SSD. Also, in my particular case, the card reader connection seems a little flaky. It’s connected to the USB 2.0 mouse port, which has taken a few hits over the rears.

    Just my 2 cents.

  6. For some reason my ASUS Vivotab won’t boot from a USB flash drive that has an Active partition, which I think makes it bootable, while connected using an OTG cable. The Vivotab comes with a microSD card slot so I’d love to be able to boot from that.

  7. I found the audio version to be tremendously helpful with understanding this cumbersome matter. Thank you Leo! Great website!

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