Apple abandoned the Mac Pro 1,1 ages ago, but the hardware is perfectly capable of running El Capitan. With a little help.
There are a number of guides available, but they tend to either a) vanish or b) not actually work exactly as indicated.
This is an attempt to detail what actually worked for me.
A gentleman known as John runs a blog that lays the issue out quite plainly. However his steps did not work for me. Here’s what I had to do slightly differently:
Use another mac to download the installer.app. You will find it in your applications folder. Copy it somewhere safe, as after it runs it deletes itself.
Install it to the drive you want to use and boot from that drive to finish the setup. Reboot with your original drive, and then enter the following commands into terminal. Replace SSD_Ext with your volume name, and john with your user name:
cd /Volumes/SSD_Ext/System/Library/CoreServices
This will take you to the first place where the boot.efi needs to be replaced.
sudo chflags nouchg boot.efi
Enter your password and remove the protection from boot.efi
Visit the following URL and download the patched boot.efi to your desktop.
cp /Users/john/Desktop/boot.efi boot.efi
That will copy the new boot.efi to CoreServices, overwriting the standard one.
sudo chflags uchg boot.efi
This will protect boot.efi again.
sudo nano PlatformSupport.plist
enter the following at the top:
<string>Mac-F4208DC8</string>
Hit Control + X, yes, enter.
This will save the file and take you back to the prompt. Now enter:
cd ../../../usr/standalone/i386
This will take you to the second location, where you need to copy the boot.efi file. Just enter the following command again (hit the up arrow to use commands you’ve entered previously)
cp /Users/john/Desktop/boot.efi boot.efi
Remove the drive, put it in your Mac Pro 1,1 (you could have used target mode as well) and reboot. Enjoy your non-crippled Mac.