Can't verify my OS
So I'm trying to download a new operating system on my mid 2009 MacBook pro 15 in. When I try to boot in recovery mode nothing happens. My screen is showing me a white screen and flashing a gray folder with a question mark in it. That happens whether I try to boot in recovery mode or simply turn it on. I've created a flash drive that has the OS on it as a boot installer. Now with that, when I try to go in recovery mode the flash drive I've created pops up and it lets me click on that and continue. It brings me to a language selection after loading with the apple logo and loading bar for a bit. Once I select the language I want it brings me to the OS X Utilities. In that it shows 4 options. Restore from time machine, install OS X, get help online and Disk Utility. I click on the install option. And it brings up the OS X El Capitan logo and the continue arrow. Click that then I have to agree to the terms. Click agree and another small window drops down asking again to agree or disagree and I click agree. Then it brings me to where I select the drive I want to download the OS to and it shows my hard drive and then the flash drive but the flash drive is in grey so I can't select it which is ok. I click the hard drive and click install and starts. Shows the hard drive I've selected with a loading bar under it but before it can start loading another window drops down, stopping the download, and says "This copy of Install OS X El Capitan application can't be verified. It may have been corrupted or tampered with during downloading." I click ok and it brings me back to the 4 options I said earlier. Please Help.
[Edited by Moderator]
Earlier Mac models