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Someone has recently installed an external hard drive on my system (Western Digital SATA, if it matters). Since the moment it has been installed (we've already removed it) I cannot start up Windows: A few seconds after "Loading Operating System..." I simply meet a blue screen of death. The error message is: STOP: 0X0000007B (0x80786A58 0XC0000034 0X0000000 0X0000000).

The error message also mentions that I should try to CHKDSK /F, which is not an actual drive on my computer, but was the one under which the external drive was. When trying to run Safe Mode, it gets stuck at ClassPNP.Sys.

I've unsuccessfully tried renaming ClassPNP, changing from IDE to AHCI, Disk Check, SFC, startup repair and system restore.

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Changing your BIOS drive type from IDE to AHCI and vice versa will result in being unable to boot windows. This action requires a re-installation of Windows. AHCI is the recommended interface type.

Without knowing which version of Windows you had running, the interface between the external drive and your computer (you said SATA, did you mean e-SATA or was it on a USB adapter?) and if an OS had also been installed to the external drive, I can only speculate at your problem. It appears likely that your BIOS had been set to boot from external media, which tried to load an OS which wasn't configured to run on the hardware that this new (your computer) device had. Earlier versions of Windows like to add drives to the Master Boot Record and reacts poorly when one is removed, often refusing to boot without them present in the case of external and multiple drive computers.

Since there isn't a question I'm going to assume you'd like help in getting the computer running again. If there is no data that you need recovered, I recommend resetting the BIOS to default, then selecting AHCI as the SATA interface, then installing a fresh copy of windows (with all unnecessary media such as USB storage and external drives removed). Once complete, disable booting from removable media in the BIOS.

If you believe you need to fix the corrupt copy of windows, set the BIOS back to the same way it was before you modified it and run the windows repair disk, or select "repair my computer" during Windows installation.

If you are unable to repair as the MBR has become too corrupt, you can use data recovery tools such as Recuva on another machine to extract the data from the desired hard disk, then perform a fresh install of windows as described above.

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From reading this my first assumption is this external HDD that was installed had Windows on it already in some form and your BIOS is trying to use that as the first bootable device. I would check your boot settings and make sure it is set to the last (or even unplug it, boot into your primary Windows HDD, then plug it back in) and format that drive.

When it says CHKDSK /F, it is not saying to run a Check Disk on the F:, it's saying you need to do a Chkdsk /f which stands for "Fix". Same thing is implied with a /R.

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