Target Display Mode on 2009/2010 27 iMacs with half defective GPU: TDM still works and boots normally with Snow Leopard
I found this article: 2009 27" Imac Bad GPU but can still use in Target Display mode 2009 27" Imac Bad GPU but can still use i… - Apple Community and I tied it myself for a 2010 27 iMac with a Radeon 4850 which is defective (boots only halfway, login only possible with safe mode but then no GPU acceleration), and I can confirm that target display mode stil
