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Troubleshooting PC. Won't boot from USB when HDD connected

I am trying to troubleshoot a PC issue for a client. I am trying to rule out a problem with the HDD and am trying to boot an UBCD USB drive.

Unfortunately when ever the HDD is connected it will not boot from the USB. If I disconnect the HDD then it will boot from the USB.

I have changed the Boot order to boot from the UEFI device which is the USB drive.

Asus H81M-E Motherboard
sandisk cruzer 4gb
Watch Question

Did you try switching the boot Legacy Only and disabling secure boot?
RaminTechnical Advisor

Disable Secure Boot from BIOS and also set Boot mode to legacy (not UEFI) and test again.



I have tried that now. However the HDD just doesn't show up anywhere when the usb is plugged in.

According to the ubcd drive diagnostics it cannot even find a controller. Not sure if the MB is bad?
RaminTechnical Advisor

Disable RAID mode from BIOS too.
Try plugging the HDD into another SATA port and rerun the diagnostic.
RaminTechnical Advisor

@Bocevski - I saw your post just right now,  it seems you were faster than me this time.
Check are all SATA ports Enabled with Automatic detection.
Distinguished Expert 2019

Try this : go to the Bios menu (Del key)
then tap the F8 key - boot menu
does your usb drive show there?  if so, select it - it should boot from it; if not, your USB drive is not bootable