Why can't I boot from an SD card?
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Let's dive right into the question.
"I have noticed many times in publications
that we can use a USB flash drive to boot 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've 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 versus an SD drive for doing these sorts of things like booting from?"
Honestly, Windows doesn't care because Windows isn't involved.
It's your computer that boots.
It's your computer that looks for what to boot from.
Windows hasn't even been loaded into memory.
In fact, it's the process of booting
that finally gets Windows into memory and running.
So it really all boils down to your
specific computer, your specific computers,
BIOS or UEFI configuration as to whether or not and what it will boot from.
So remember what it means to boot is
the process of identifying a device that has a bootable operating system on it.
It then uses a small program on that device to load a larger program
from that device to load potentially a larger program from that device.
To finally load Windows. It's a sequence of steps that causes
the operating system to be loaded from wherever it happens to be stored.
It's kind of a chicken and egg thing because, of course, like I said,
Windows isn't running yet, but you want to load Windows.
So what ends up happening is the BIOS.
I'll use that term for both BIOS and UEFI.
Does a sequence of checks to find out where the operating system happens to be.
You will be able to in most BIOSs change the order that this happens in.
I'm going to go through a simple example, but the order of this example is perhaps
the most common, but it's also one that would prevent you from booting from USB.
For example, when you turn on your computer or when you
reboot your computer, the BIOS essentially asks a series
of questions that help it to determine what to do.
First, it might say, Is there a hard disk?
Is there an operating system on that hard disk?
Great.
We'll boot that. Done. No. Then is there a CD or a DV drive?
Is there an optical drive? Is there a media inserted in that drive?
Does that media have an operating system on it?
Great. We'll boot that.
If not, is there a USB drive?
Does the USB drive have a bootable operating system on it?
Great.
Let's boot that. Finally running out of all other options, it'll say, you know what?
I couldn't find anything to boot from, no bootable device found.
So that sequence allows you to have an operating system in different places,
and then it's the sequence that the BIOS checks.
That basically gives you the opportunity to say, you know what?
I'm not going to have a hard disk.
I'm going to have a boot from USB,
but of course, there's almost always an operating system on the hard disk.
So if you want to alternately change or
boot from USB, you need to change the order.
What that means is then, in the BIOS, you change the order of check so that it
checks the USB devices first or checks the DVD or optical drive first,
and you put the hard disk later or slower in that list of checking that then says
the BIOS will then say, you know, what, is there something on USB?
Great. We'll boot from that.
If there's not, then we'll boot from the hard disk.
If there's something on that now,
that can still be a problem because some BIOS get it wrong.
Some BIOS will not check for an operating system.
They'll just see if there's a USB device
and if there is, they'll try and boot from it and fail.
Which means on those machines, if you have the USB device before the hard
drive and you have a USB device plugged in, your machine won't boot.
The symptom is very simple if you unplug
your USB external hard drive or card reader or whatever.
If all of a sudden the device starts
booting properly from the hard disk, then you've got one of those bios'with.
This issue. On the other hand, most of the time,
most machines these days don't have that problem.
Now I keep saying USB device.
Note, I didn't say a specific type of USB device.
I didn't say an SD card reader or a thumb
drive, or even an external optical media drive, which is very common.
Unfortunately, this, too, depends a lot on the UEFI or BIOS
configuration or even its own capabilities.
Sometimes it would be plenty.
It'll be just fine to have your SD card have a bootable operating system on it.
Have that be in a USB SD card reader and be able to boot from it.
As a side note, most internal SD card
readers actually masquerade as USB devices.
Even though they're built
into the machine, they have an internal USB, so it's usually the same interface.
The same issue. Regardless of whether the SD card reader
is built into your machine or an external device, sometimes it'll work.
Sometimes it won't.
My recommendation is that if you've got the opportunity, make your life simple.
Just use a USB memory stick, because if it's going to be able to boot
from USB, it'll be able to boot from a USB memory stick.
The other options, be it an external CD or
optical reader, or even an external SD card reader, they might work.
And if it's worth it to you, experiment.
But the bottom line is that if you really want to make sure that something's going
to work, especially across a wide variety of machines, just use a flash drive.
Just use a USB flash drive, and it will make your life that much simpler.
Like I said, there's so many unknowns here and there's so many specifics that it's
just not something that we can give a blanket answer for.
With the exception of if you're going
to be able to boot from USB, it'll be from a USB memory stick.
Hope that helps hope.
That helps clear up a little bit of the confusion.
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