Creating a bootable El Capitan USB drive

Try booting the iMac into Internet Recovery Mode (Command + Option + R) to see if you can access the online macOS installer. It is best to connect the Mac directly to the router using an Ethernet cable for a faster more reliable connection.

Don't think you can create a bootable USB on a Power Mac G5 the last operating system

it could run was Mac OS X Leopard 10.5.8 which can't run the createinstallmedia command in Terminal.

I think Mac OS X Yosemite 10.10 was the first to offer the possibility of creating bootable installers.

A lot of people have tried making bootable USB installers with Transmac on Windows machines

some with success, more without.

Borrow a mac from some one if your macs hard drive is not working.

If you want to create a bootable USB flash drive installer for Mac OS X El Capitan,

follow these instructions.

How to get old versions of macOS – Apple Support

Go to Download OS and click on OS X El Capitan 10.11

this downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running El Capitan.

This includes macs that have the potential to run El Capitan but have been upgraded to a newer OS.

A mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the InstallMacOSX.pkg to the Install OS X El Capitan.app which 

you will find in your Applications folder, it should be 6.2GBs in size.

(If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install El Capitan on, that is for later.)

To start the installation of El Capitan double click on the Install OS X El Capitan.app.

The copy of the install app self deletes after installing El Capitan, so make sure you keep a copy of the 

InstallMacOSX.dmg if you need it in future, or you could just make a copy of the Install OS X El Capitan.app 

prior to installing and moving it to an external drive for safe keeping. 

It is also possible to create a bootable USB installer disk using the Install OS X El Capitan.app in the Applications 

folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

If you want to make a bootable USB installer for macOS High Sierra you must open

your Safari app and then click on the How to get old versions of macOS link

above. Go to the downloads section and click on macOS High Sierra.

This will redirect you to the App Store High Sierra page, it should say

Get or Download. This downloads the Install macOS High Sierra.app

directly to your Applications folder.

Use this and the instructions in the link above to create a bootable USB installer.