HP Z420 USB ports inaccessible

[cripes]

Hi

This is a really strange one, i have a HP Z420, and the USB ports on the rear look fine, but when i go to plug in a USB cable it just doesn't fit. I assume that the casing is somewhat skewed, but it doesn't look like it's been 'tampered' with.   It must be the casing panel - as i can't fit a PS2 cable into the mouse slot, but the keyboard one it fine.

Does anyone have any experience of how to maneuvre this metal plate on the back?

[cripes]

Also, just in case someone has been through this with the Z420 i'm trying to install Windows 7 (CD) on a fresh HDD, but i get the "a media driver your computer needs is missing" error.  The microsoft guidance says the solution could be to transfer the image to a usb drive - but that makes no sense, i have used this disc to load the os in the past i don't see how loading it to USB would be different.  I have CD/DVD as first boot, and disabled USB 3.0 to see if that was the issue - but no change.