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Debugging USB boot support helps PC pass POST

An external USB hard drive with some kind of bootable partition can be connected to a PC and booted directly. But if the boot support for a USB device is not thoroughly debugged, this could prevent the PC from coming out of POST.

Web developer Scott Hanselman recently blogged about an issue that potentially affects any system with an externally connected USB hard drive.

His story is a familiar one. Something goes terribly wrong, leading him to believe the system itself is dead meat. Then he discovers the problem to be something so innocuous he'd never have suspected it.

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After struggling (in Windows Vista) with a device that refused to be recognized, Scott eventually ended up with a system that wouldn't even come up out of POST. He finally traced the problem back to his Iomega 35GB USB drive, which was still plugged in and turned on. After disconnecting the likely culprit and cycling the power, all was well again. (So much for blaming the problem on Vista, tempting as that was.)

Why did this happen? For one, most USB hubs that are based directly on a PC (as part of the motherboard, or as an add-on card) now support USB-connected boot devices of some kind. This means that an external USB hard drive that has some kind of bootable partition can be connected to a PC and booted directly, without being mounted in the PC itself.

The problem? Sometimes the boot support for the device is not as thoroughly debugged as it could be. There could be a bad interaction with certain USB host controllers, and the device causes the host system to lock up or behave unpredictably.

How to debug USB boot support
Boot support is a fairly low-level system function, so anything that hooks into it needs to be well-behaved. In Scott's case, his external Iomega drive seemed to be troublesome enough that it was interfering with his system's boot process (although this might not be due to the drive alone, but a misinteraction between the drive and that particular PC's host controller).

Note: If such a device is plugged into an external hub rather than directly into the PC, it will not boot. On the other hand, it probably won't cause boot-time problems either. If you're experiencing such problems and you still want to have the drive available after boot time, move it to an external hub.

About the author:
Serdar Yegulalp is editor of the Windows Insight, (formerly the Windows Power Users Newsletter), a blog site devoted to hints, tips, tricks and news for users and administrators of Windows NT, Windows 2000, Windows XP, Windows Server 2003 and Vista. He has more than 12 years of Windows experience under his belt, and contributes regularly to and


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