Welcome to LQ.
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You need to access the BIOS/Setup.
Press F2 when the Dell logo appears after switching on. Press every few seconds until the message Entering Setup
First, look for Onboard Devices > USB Controller. This setting must be set to ON
Next, make sure that you have you set the correct Boot Order
in the BIOS/Setup.
Look at this video:
and make sure that the Boot Order is set exactly the same as this:
1. USB Storage device
3. Internal HDD
Various other non-numbered items underneath (ignore)
Note that these 3 items MUST be numbered
in order for them to be BOOTABLE.
Press the Spacebar
to number/make the drives bootable.
When you have the correct numbered selections as above, press the Esc key.
Personally I would use a DVD, but if you prefer to use the USB drive that is fine.
With your DVD or USB drive inserted, press F10 to Exit and Save Changes.
Hopefully the PC will now reboot to your Linux installation.
If this fails, you may need to use a Boot Repair Disk.
Download the .iso from here:
and then burn to CD.
See “Alternative Method” here for instructions:
By the way, which version of Ubuntu are you using? (e.g. 14.04 32/64 bit)