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Why can't my machine boot if my external drive is plugged in?

Why does my desktop stop booting up with an external 1TB USB drive attached
and switched on? It boots normally from the normal internal 80GB drive with the
external 1TB drive switched off!!

This happens surprisingly often – I’ve heard many people mention it.

Typically, they’ll just leave the external drive powered off until the
machine has finished booting; at which point, they turn it on and Windows
recognizes it.

Depending on how you use your machine, that might be the most appropriate
solution.

On the other hand, it’s also possible that it’s a very simple fix.

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Change the boot order

The problem is simply that your computer’s BIOS is looking to boot from an available, bootable USB-connected device before it looks to the hard disk from which it normally boots.

The solution is to change the boot order in your computer’s BIOS so that it looks at the hard disk before USB devices or doesn’t look at USB devices at all.

Unfortunately, I can’t tell you exactly how to do this – the BIOS controls your computer before Windows starts and is unique to your machine (though often common across product lines or even manufacturers).

You’ll need to check your system documentation to determine the exact steps. In general, when your computer boots, there’s a key that you need to press right away that causes the BIOS to enter its configuration mode instead of booting Windows. Then, somewhere in there should be an item called “boot order” or something similar. In that, you’ll want to make sure that your hard disk is checked before the USB devices if you want to boot normally while that external drive is connected and plugged in.

What’s boot order?

Your computer’s BIOS is programmed to look for a device from which to boot. It typically works something like this:

  • Is there a floppy disk inserted? Try to boot from that. (Even if your computer has no floppy disk, it’s very likely that your BIOS still includes this option.)

  • Does the hard disk have a recognizable boot sector? Try to boot from that.

  • Is there a CD or DVD inserted and does it have a recognizable boot sector? Try to boot from that.

  • Is there a USB device inserted and does it have a recognizable boot sector? Try to boot from that.

  • If none of the above apply, give some kind of “no bootable device found” error.

The boot order in the example above is: floppy, hard disk, CD, USB. As mentioned above, you can typically alter the order in which the BIOS checks for devices from which to boot.

Things get a little interesting because of this phrase, “Does it have a recognizable boot sector?” As I understand it, there’s really no standard way to answer that question. In your case, not only is the USB drive earlier in the sequence than your hard drive, but if the BIOS is checking for a boot sector on that device, it seems to think that it’s found one, even though there’s nothing on that USB drive to boot from.

If you occasionally do boot from USB

Here’s where things get sticky.

If you actually do have other USB devices from which you occasionally want to boot your machine, you probably do want to have the BIOS check USB devices first.

And not just before your hard disk, but before that other USB device, your external hard drive.

I don’t know of a way to control the order that BIOS checks the USB devices if there’s more then one.

So you’re left with the workaround that you’re using now:

  • Let the BIOS check for bootable USB devices before it checks the hard drive.

  • Turn off your external hard disk when you boot (be sure to click Safely Remove Hardware if you’re rebooting and Windows is still running), and turn it on again after Windows is running.

It’s messy, but it works.

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14 comments on “Why can't my machine boot if my external drive is plugged in?”

  1. Different computers use different keys to get into the BIOS configuration, but the most popular choices are Del, F2, ESC, F1, F10, F12 or F11.

    Reply
  2. HP and Dell computers have a Function key option to select the Boot menu. From the menu you can select the device from which you would like the computer to boot. F9 for HP, I can’t remember the key for dell but it does show on the initial splash screen. I use this capability in my Tech work all the time.

    Russ Martinson

    Reply
  3. Hi,
    I had a similar Prob. only in my case my computer would not load if the printer was on. HP support could not Help.I tried all the changes in the Bios no go. After changing the mother board and with the Bios, all my problems went away, and not only that but a few other hic ups disappeared.
    joe

    Reply
  4. The boot problem when an external disk is attached occurs only if this external disk has an active partition without a BCD and an OS. It finds the active partition in the MBR but runs into the ‘desert’ because there is nothing to load.

    Go into Disk Management and deactivate the partition. Then the problem should be fixed.

    Reply
  5. I have a similar problem where my boot hangs until my machine stops searching for something on one of my (data only) external drives (as indicated by the activity light flashing). Sometimes, it goes on and on for 10+ minutes. VERY frustrating.

    Reply
  6. The majority of computers allow you to access the BIOS by pressing the F12 button at boot up. Other variations could include F2, DEL(ete), or in the case of Lenovo machines, press ENTER on boot up to halt the boot process, and access a menu of options.

    I generally configure machines to boot from CD, USB, and then Hard Drive as the third option. It delays booting in to Windows for a few seconds, but if ever I’m in a situation requiring me to recover data, I can boot to an external USB drive and recover what ever I need, before rebooting from DVD recovery or installation media.

    Reply
  7. I had a similar problem on my old (circa 2004) Compaq XP machine. After connecting a 1 TB or 2 TB USB drive (can’t remember which, I have both), I found that the PC would not boot. Disconnecting the new drive, it would boot OK. During the troubleshooting, I set the BIOS to display the boot log as it was booting (instead of just the Compaq logo). This showed that the hangup occurred when it was scanning for USB peripherals (this was before booting Windows even started).

    After some further research, I found that the cause was that my BIOS could not handle USB disk capacities of 1 TB or over 1 TB (don’t remember which). I found a newer version of my BIOS and updated it (with some trepidation — never updated a BIOS before and was aware that if the update failed, I could be left with a nice boat anchor!). I am not sure at this distance whether the new BIOS fixed the problem (but I think not), because the whole PC went to God shortly thereafter and I replaced it with a shiny new Windows 7 machine. No such problems on the new machine.

    So this is another possibility — do you have an old machine or BIOS from back when USB disks never got as big as 1 TB? Check the boot log to see at what point it hangs. If it does not pass the USB device scan, then you may have the same problem I had.

    Reply
  8. If I have my mouse plugged into my USB hub, my computer won’t boot, even if USB booting is disabled in the BIOS. Either Windows or Ubuntu will start to boot, but then hang, and never reach the login screen.

    You better believe I lost a lot of hair and cussed a lot while troubleshooting this one, back when I first got the computer. The only solution that I could find: don’t plug mouse into hub.

    Reply
  9. I have a 9-year-old Dell 4600 with a 250 GB external HD used as backup plugged into a front USB. Never had trouble leaving it plugged in until last week when “something” began causing the computer to hang–first in the middle of whatever I was doing, making me have to turn it off at the power button, and then being unable to boot at all, until I used the Windows OS disk. Looking for things I could disable, I unplugged the external HD, which seemed to help. Once the machine stayed running long enough for me to do a system restore, operation returned to normal. I don’t know what made it start hanging in the first place, but I will remember this article next time it happens.

    Reply
  10. If it is not your computer’s BIOS then replace the power supply. You need more power to boot and if there isn’t enough power the boot process will stop.

    Reply
  11. I agree with Lanier. My old desktop will hang on boot if too many usb sticks or more than one external drive is connected – all my ext. drives are 2.5in. thus require pwr from pc.I put this down to either an underrated suppy or ancient mainboard designed before all the modern stuff we plug in.

    Reply
  12. I’ve noticed that if a USB-3 device is plugged in to my pre-USB-3 computer, it will interrupt the normal boot process, whereas a USB-2 device will not cause a problem. However, the USB-3 stick is usable for reading and writing if I insert it after the machine is already booted.

    Reply

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