Mac Pro 2008 "Option" or Boot Manager wont access or not working after chime {need help}

[TechJhoe.]

so i have an issue on my mac pro 2008, i accidentally reformat my desktop because of very laggy performance, now im trying to reinstalled my OS X using recovery mode but after signing in my apple i.d there is an error that cannot verrify my email adrress, so i buy a new SSHD to install fresh OS X but i have the same issue. now i download a el capitan OS X on the internet to make a bootable USB, but while installing there is an error saying the "corrupted files" and i cant access the boot manager pressing "OPTION" key after the chime

please i need help to recover my desktop

 

[h9826790]

If ecovery partiton won’t work, and you can’t install OSX on an external USB 3.0 HDD by another Mac, burn an OS installation disc usually can make your life easier.

For boot manager, which GPU you are running now? What do you mean “not working”, black screen or white screen?

 

[TechJhoe.]

If ecovery partiton won’t work, and you can’t install OSX on an external USB 3.0 HDD by another Mac, burn an OS installation disc usually can make your life easier.

For boot manager, which GPU you are running now? What do you mean “not working”, black screen or white screen?

Black Screen only and on my monitor it detect no signal until it sleep. i tried also to used bootable usb but the problem is i cannot access the boot manager "OPTION Key" after the chime.

i can access only is the recovery mode which is Command R but there is also problem while on installing OS it says corrupted files but i tried many el capitan installer same problem please help

and for the GPU the ATI HD 2600 XT are not working only the NVdia 1GB card i forgot the model number.

is there anything wrong if my GPU has a problem while installing new OSX