Mac: USB install not working My Mac Pro become bit slow, I installed a lot of things, I downloaded a lot of torrents but I did not find any particular problem, but I found it is better to reset all my Mac. So I went through steps prescribed on YouTube: 


Shutdown Mac.
Press Command + R and power button. 
I got a window contain, disk utilities, so I did erase all my Mac disk. After
I selected “Reinstall OS X”
After a couple of steps, It is asking for my Apple account user/password. I have given both. But it says “this service unavailable, please try later. ” I tried more than 20times, Same user/password I am using on my mac air also it working well.
For this issue, I find a solution online, that first login in iCloud or iTunes with Safari. After try again. I did same but no luck.


So, I plan to install with USB. For that, I followed different video from YouTube (https://www.youtube.com/watch?v=bIbOg89KIUE).
That says


Take 8GB or more USB flash drive.
Reformat it as bootable USB, that I did.
Download “Install macOS Sierra” from App Store.
Copy it to USB drive. I did all that as he showed in the clip.
And connect that USB drive to the mac pro. 
Press Option + Power button
I got two options on Mac internal hard disk second my USB drive,
I selected USB drive and press install.
Then I am not getting any thing just white screen, I waited for 10mins no response.
For this problem, I found a solution online, that says, give very generic permissions to that USB. I did and tried again even no luck.


Please help me how to reinstall my Mac Pro please.
My advice would be to boot back into OS X. Erase the USB stick and follow the commands given here using Terminal https://support.apple.com/en-gb/HT201372

This is Apple's method of doing it and I've found it far more reliable than using Disk Maker X or any drag and drop method. For clarity if your USB stick was formatted as Untitled, the command would look like this:

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app


Because you started the command with the word 'sudo' you will be asked to enter your user password, go ahead and do this and then wait. It can sometimes take up to an hour for this process to finish.

Next make sure have a backup before you proceed to do anything. Time Machine to a USB hard drive of equal or greater size than your hard disk will be good enough.

Once the above steps are done, restart your machine with the memory stick inserted and turn the machine on while holding the alt(option) key. 

Select the Mac OS Installer and follow the instructions. If you're going for just a reinstall over the top of your existing files, simply select re-install Mac OS and let it run.

If you're going for a completely clean install (recommended if you're having issues) make sure you have backup of your system first then proceed to use Disk Utility to format your main hard drive (best practice is to call it Macintosh HD). Then go back to the installer menu and re-install Mac OS to the newly cleaned drive.