Ubuntu netboot USB not appearing on iMac Startup manager

Apologies if this question is inappropriate for this forum I am not sure whether I should be on the Apple or Ubuntu StackExchange.

I am trying to turn an iMac mid 2010 21.5" into a home server by installing Ubuntu Server 20.04 LTS on it. Startup manager is not recognising the bootable USB that has been loaded with the Ubuntu Server Netboot ISO, though it is recognising a USB that has been loaded with the standard Ubuntu USB - does anyone know why this might be? I am using the same program to create both bootable USBs and I have tried different USB ports.

For context there isn't a functioning OS on this machine. An earlier attempt to install Ubuntu Server crashed so when I boot the machine it goes to blank screen with just only '_'.

The standard Ubuntu ISO can't be installed due to a GPU issue: 'no UMS support in Radeon module' is the error that I receive and I have tried editing the GRUB settings to include 'nomodeset' as suggested on forums such as this one: E.g. Installing Linux on 2009 iMac, black screen after bootloader

1 Answer

The short answer is the mini.iso is bootable, just not bootable from a USB flash drive on a Mac of your vintage.

Here is some of the things I tried.

I transferred an ISO file to a USB flash drive by writing the file directly to the flash drive. So, what appears on the flash drive is exactly what exists in the file. There many ways to accomplish this. I used a Mac booted to macOS. The identifier for the flash drive was disk2. An example of the commands is given below.

diskutil unmountdisk disk2
sudo dd if=~/Downloads/mini.iso of=/dev/rdisk2 bs=1m

The same can be accomplished with Linux by entering the commands given below. Here, the identifier for the flash drive was sdc.

sudo umount /dev/sdc*
sudo dd if=~/Downloads/mini.iso of=/dev/sdc bs=1M

I found the USB flash drive created using the mini.iso file was BIOS bootable by the Startup Manager on a 2013 iMac. While I was able to boot, I choose not to install. The same flash was not detected by the Mac Startup Manager on a 2007 iMac. This is expected, since iMacs could not BIOS boot from flash drives until at least after the 2011 model year. Neither Mac Starup Manger could detect a EFI boot file on the flash drive, even though such files do exist on the flash drive. I state this because the flash drive can EFI boot in a VirtualBox virtual machine on the 2013 iMac running Catalina. I was able to install Ubuntu 18.04.5 LTS on the virtual machine.

I used the same method to transfer the ubuntu-20.04.1-live-server-amd64.iso file to a flash drive. This was detected as EFI bootable by the Mac Startup Manager on the 2007 iMac. I was able to boot and installed an Ubuntu server. A present, the Mac Startup Manger is needed to boot Ubuntu server on the 2007 iMac. I have not tried fix the Mac to boot Ubuntu server as the default operating system. I should also point out that this version of Ubuntu server boots to a command line interface (CLI).

Next, I tried using the Startup Disk Creator that comes with kubuntu. This basically does the same thing as the dd command used above.

When installing Ubuntu Server, I made the following selections.

• I chose to update to the new installer.
• I chose to not use the entire disk without a LVM group.
• I chose to install the OpenSSH server.

When I finished installing, the content of the USB flash drive had changed. A compare with the original ISO file found differences.

Update

I was able to install Ubuntu with a desktop graphical support on the 2007 iMac by using the mini.iso file. This was accomplished by using the dd command to copy the mini.iso file to the internal drive (/dev/sda). Next, boot to the Mac Startup Manager and select the Windows label under the internal drive icon. Once the Ubuntu installer starts, you can overwrite the the contents of the internal drive with the new Ubuntu installation.