Error 403:forbidden when trying to reinstall OS X El Capitan on 2009 Mac Pro

So...your options are limited and without another way to make a bootable USB...

Theoretically - you might be able to do the following, although I've never done this myself and make no claims at all if it will work. But you have no computer at all right now, so I guess you can't make it worse...

Assuming you have your current 2009 Mac Pro able to boot into Recovery mode and 2 x USB flash drives of sufficient size with nothing on them you don't mind losing:

• Boot into Recovery mode;
• Insert the two USB drives;
• Open Disk Utility:

1. Erase the two drives, naming one Download and one Installer.

• Close Disk Utility;
• Log into your Wifi;
• Open Safari:

1. In Safari Preferences, change the download location to the USB named Download;
2. Download the El Capitan installer app (not a dmg) someone saved to the Internet Archive...there's another thread around her somewhere where someone posted a link.

• Once the Install OS X app is downloaded, close Safari
• Open Terminal

1. Run a modified version of the bootable installer creation command, given that your Install OS X El Capitan app is in a non-standard location. Given you are in Recovery, I believe you are in Single User mode so sudo is neither needed or probably available...but I think the PS1 prompt in Recovery's terminal is # so you're root...

/Volumes/Download/Install\ OS\ X\ El\ 
Capitan.app/Contents/Resources/createinstallmedia --volume 
/Volumes/Installer --applicationpath /Volumes/Download/Install\ OS\ X\ El\ 
Capitan.app

If this works, you will have an installer you can then boot into and run the install from...

If it fails, it's probably going to be because Recovery mode won't allow you to execute programs from removable disks...or there's a lack of permissions

Personally, seems like more trouble than it is worth - I'd try to find a friend with a Mac that could help out instead.

Cool.

Just so you know, there's a lot that just doesn't work on El Capitan any more...so you may want to consider:

1. Anti-virus. Most people in the community forums disdain AV on Macs or maybe recommend Malwarebytes. Malwarebytes no longer provides an older Version 4 that supports El Capitan. ClamXAV 3.7 will work.
2. Web Browser - Safari is mostly broken, even on (especially on) Apple's own sites that use very modern web standards. Use Firefox 78.15.0 and Opera 89.0 (sites that don't work on one might on the other in my experience).
3. VPN - another thing that most here disdain. But if you need one, then you need to find one with a version that both supports El Capitan and OpenVPN 2.4...the only one I've found so far is Viscosity 1.8.4, but it is just a VPN client and you'll need to provide your own VPN configs from a VPN Server provider (for that I would probably look at Proton VPN, Mullvad or similar). No guarantees regarding any unpatched security holes in that old horse though...but you have a Mac Pro, so probably don't need that since you won't be in coffee shops with it.
4. If you want a bootable clone, you're going back to Carbon Copy Cloner 5.1.28. Not sure what versions of other cloning apps might work.
5. And if you ever have an issue with the computer and need to post here for diagnostic advice...you'll need to ask Etrecheck (one of the long term users here) for a download link to Etrecheck Pro 6.4.3, since he no longer provides El Capitan compatible versions from his website, as far as I can tell.

Good luck.

Well, I got curious to know if the theoretical is practical...

Yes, the method outlined above will work - but there is a big caveat. You have to have a complete full installer app.

What does that mean? Well, there are lot's of "Install OS X El Capitan.app" copies out there - in fact, there is one at the root of the system volume in Recovery mode - you can't use it to make an installation disk because it is a skeleton app missing some important code. In fact if you try you'll get an error telling you that it doesn't appear to be a valid installer.

The "Install OS X El Capitan.app" that ends up in /Applications when you download the .dmg from Apple and run the .pkg installer inside is a complete application and you can build installer disks form it.

So, you need to get the right copy of "Install OS X El Capitan.app" off the Internet. I have no idea how to tell if any given copy on the Internet Archive is the right one (I faked it by extracting a good version off an installer I already had and running the build in Recovery mode - downloading multiple copies of a 6.22 GB file to check them was way more effort than I was willing to expend).

But the gist of it is, get a good "Install OS X El Capitan.app" download from the Internet onto a flash drive and have a second flash drive you can make the installer from and a working Recovery partition you can boot into...and making an installer is entirely possible.

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