I edited the
/etc/default/grub file's command line parameters, so that it reads:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash radeon.modeset=0"
(your system's parameters may vary. Just look for the
GRUB_CMDLINE_LINUX_DEFAULT value, and add
radeon.modeset=0 to the end, before the close-quote)
Saving the file, running
sudo update-grub made this the default boot instruction.
Good to know: setting "
radeon.modeset=0" disables any attempt by the Linux OS to use the GPU's hardware acceleration. I think this is what Suzamax meant when claiming that Linux is "not going to work properly" on these old Radeon iMacs.
I would not say this makes Linux a bad choice for these old machines in general, however. I'm writing this on a 2010 iMac that Apple no longer "supports" with the latest MacOS or Safari. Basic usage had broken down, to the point where even ordinary web tasks like youtube videos were failing to load properly, due to outdated browsers with no modern HTML5 support.
Long story short, losing graphics acceleration in a nearly decade-old machine is a fair trade-off. This computer is not going to be used for high-end gaming and video applications.
Meanwhile, the core i3 CPU scores higher in Passmark than most brand new "budget" PC's available right now (which also have no discrete GPU -- purchasing a new low-end laptop would mean the end-user gets the same lack of GPU graphics processing, but they'd also get LESS CPU speed and less RAM, and they'd have paid new money for the lower-utility product):
I always recommend keeping old machines with strong CPU's and SATA connectors (for SSD upgrade) in use, rather than prematurely turning them into e-waste.
To that end, Linux is WONDERFUL for these old iMacs, because it's not like anything else is going to run on them. Apple made sure of that. The only other place this machine was headed was a landfill.