Unable to downgrade to High Sierra ?

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Hello everybody !
First, thank you in advance to apologize for my poor American language, I'm French…

Here is my problem :
I own a MacBook Pro 15" mid 2009.
I managed to install Mac OS Catalina using patcher (from http://dosdude1.com/).
Then I've decided to downgrade to High Sierra, but without success until now.
I managed to downgrade to Mac OS Mojave using patcher (from http://dosdude1.com/mojave/), SSD format Mac OS Extended (Journaled).

I've created external drive boot High Sierra with Terminal commands, diskmaker software, MacOS High Sierra Patcher, and nothing work !
I know that I have to modify date (to 2017/2018).
When I try to boot from external drive I've got gray circle (with /) ? And this drive is correctly bootable onto my iMac 2014.

If anyone could guide me I would be very grateful !!
Nicolas (from France)

 

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I've also tried keyboard shortcuts : cmd + R, cmd + opt. + R, cmd + opt. + **** + R
without success. Boot sequence always to Mojave internal SSD.