….if the iMac is running OS X 10.7 Lion or later, I think you can do the entire job, including erasing its HDD/SSD in Recovery mode.
Since that won’t be the process for me (currently 10.6.8), I assume I just poke the thumb drive, containing the El Capitan download, into a USB port and start the machine with the Option button pressed. Then I should be able to select the thumb drive and it will take care of erasing the drive and installing the system. Yes?
No... The El Cap d/l won't be bootable, so you'll have to plug in the thumb drive and double click on its icon to launch the installer, which will not erase the HDD/SSD.
You can make a bootable installer, but I'm not up on the details.
I'm not sure, though, that you can upgrade Snowy to El Cap without a pit stop in !0.7. Didn't you have a problem earlier? (More: Looks like you can.)
Personally, I'd
really prefer to see the iMac's HDD/SSD wiped (in FireWire target disk mode), but unless you can get into Recovery, you'll need an adaptor (
this one, I think) to do it.
More: I just took a look, and you may actually be able to upgrade to El Cap via Software Update.
... maybe
