Why is my gamedrive ssd not letting me play games on my series x

Series S/X optimized games can only run from the internal drive or one of the expansion cards created for those consoles. You can still use the SSD you have to store the games or store/play backwards compatible games.

So only S/X games I can’t play but other games I can?

It's because the Series S/X are new gen. They require the xbox SSD. You are using an HDD. SSD's are way faster than an SSD, the series s is trying to run the 120fps and higher graphics. Thats why they are telling you to move the games. It wants you to play them on the internal SSD storage.

Not really...

Xbox Series X games / upgrades / updates has to be installed onto the Series X internal or expansion card SSD,

You can have Xbox One games on your external hard drive that will get an Series X upgrade but the games will need to be transferred to the Series X and then the Series X upgrade will need to be downloaded onto the Series X SSD,

As an example...

I have Marvel Avengers on my external hard drive that will get the Series X upgrade and I will transfer the game over to the Series X SSD ready to download and update the game to the Series X version when available.

Fair enough, a slow internet connection is as good excuse as any.

Be aware however that while that drive will work on Series X, you won't be able to play Series X games directly from it. Series X games will require faster access and read speeds. My understanding is that the Series X games can be 'backed up' on an external
drive and then need to copied/moved over to either the internal SSD or the External SSD's that will be available for the machine.

The Series X will read and play Xbox One games from an external drive though.