mac linux usb installation // firmware refused to boot

[nas]

Hello,
I have an iMac 5,1 (late 2006 model 2.16GHz Intel Core 2 Duo running OS lion).
I've installed refind and I am trying to install ubuntu linux 14.04.3 LTS (32bit version) from my USB. This is the message that I receive after chosing the USB to boot from:

---

*Starting legacy loader
Using load options 'USB'
Error: Not Found returned from legacy loader
Error Not Found from LocateDevicePath
Error Not Found from LocateDevicePath
Error Not Found from LocateDevicePath
...
Error: Load Error while (re)opening our installation volume

The firmware refused to boot from the selecteed volume. Note that external hard drives are nt well supported by Apple's firmware for legacy OS booting.

Hit any key to continue
____

If I press any key, nothing happens, the image is frozen and I have to shut down my iMac by using the power button.
I have visited many sites so far but I haven't been able to find a solution although I have tried several of them. I've tried to install ubuntu through live cd but I received a frozen black screen or another one with 2 boot cd options but I was not able to make a choice.... I've also tried the suggested method from the ubuntu site for making a bootable usb or alternative ones that I found in the web but the problem remains.
Please note that I am a newbie and not familiar with what exactly I need to do in order to configure the refind file settings to run smoothly.

Can somebody help me please?
Thank you

[Roderick W. Smith]

Unfortunately, legacy-mode booting from external devices has always been a bit unpredictable with Macs. I have two suggestions (to be tried individually or together):

• Use the Mac's own boot manager, rather than rEFInd, to boot the external medium. This can be done by holding down the Option (or Alt) key on your keyboard when you hear the startup chime and selcting the external medium.
• Try booting from an optical disc rather than from a USB flash drive. Most recent Ubuntu images won't fit on a CD-R; you may need a DVD. (I haven't checked the 32-bit 14.04 images specifically, though.)