The Boot Manager on an iMac is call the Startup Manager. This resides on in the firmware on the logic board. The erasing your SSD has no effect the firmware. You can access the Startup Manager by holding down the option on startup. This startup manager only recognizes certain operating system configurations, such as:
- Properly installed OS X and macOs operating systems.
- Properly installed BIOS booting operating systems such a Windows.
- Operating systems with a bootable
bootx64.efi file stored in the
EFI/boot folders of a FAT formatted EFI partition.
Note: Other operating system configurations can also be booted on iMacs, but not from the Startup Manager.
Your model was built before Apple created the ability to download OS X/macOS from the internet. Therefore, you can not use the internet to directly download and install OS X or macOS on your model iMac.
- Use the installation DVD that came with your iMac
- Buy a Snow Leopard DVD. Once Snow Leopard is installed and upgraded to 10.6.8, you can download newer versions of OS X/macOS from the internet.
- Find another Mac where you can create a bootable USB installer. See: How to create a bootable installer for macOS. The newest version of macOS that you can install is El Capitan (10.11.x).