Erase disk fails

Disk Utility will not erase the disk you booted from. To erase the entire Macintosh HD you will need to boot from an external drive. For example, an external bootable drive, a USB installer, a the recovery partition on a Time Machine drive, etc.

The erase fails instantly as “Couldn’t unmount disk” appears in the Details text box. Why does this happen and more importantly, what can be done to resolve this?

That is working as designed. How would you erase a disk that is being used?

Since a 2009 would not have a recovery partition, you can also use the original install DVDs that came with the Mac. Boot into that, erase your drive, and reinstall the original OS. You can then upgrade to El Capitan.

The other disk images are part of the OS and you should not and cannot use them - they are needed by the OS.

As I said, internet recovery is not available on a 2009. And, no, you cannot boot from install DVDs of another computer because, in those days, the install media was machine/build specific. You can try and see if you can boot with either Lion or Mountain Lion - that may work, but it depends on whether or not you followed directions to make a bootable installer. If not, it probably won't work either.

You can try BDAqua's suggestion, but I'd think it would be best to plug an external drive or USB stick into that Macbook because I do not know if it will allow to download an older (El Capitan) OS to a machine running High Sierra. That way, you could direct the download to the external drive.

Don't use Firefox for this...

https://itunes.apple.com/app/os-x-el-capitan/id1147835434?ls=1&mt=12