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
Raoul EdmondsAsked:
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Did you try switching the boot Legacy Only and disabling secure boot?

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RaminTechnical AdvisorCommented:
Disable Secure Boot from BIOS and also set Boot mode to legacy (not UEFI) and test again.
Raoul EdmondsAuthor Commented:

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?
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RaminTechnical AdvisorCommented:
Disable RAID mode from BIOS too.
Try plugging the HDD into another SATA port and rerun the diagnostic.
RaminTechnical AdvisorCommented:
@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.
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
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