why ca't I run catelina on my 2009 iMac tech answer

+1, if you are unfamiliar with Metal please click Metal - Apple Developer or https://en.wikipedia.org/wiki/Metal_(API).

In addition, no your 2009 cannot be upgraded to run a Metal enhanced GPU. If you have a need for Catalina (please note correct spelling) you need a 2012 or later iMac or another Mac (Mac mini, MBA, MBP etc..) that supports Metal.

Because it requires a Metal-capable graphics card.

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Your welcome.