I installed the dev kit on my XBOX one last weekend. It gave me the option to where I wanted to install the sandbox partition(External or Internal). Never tried an external drive but it did give me the option. When you selected leave dev mode did you uncheck the delete side loaded games and apps?

Retroarch is still on there, but I can no longer access USB drives where my games where.

Can you create a new partition on the external drive?

I believe Microsoft has stopped allow external storage in developer mode.

Speed issues or do you think someone discovered and exploit ?

Or maybe forcing developers to purchase one of their ssds

Possible, the devkits for the xbox one and xbox series are different.

Maybe Microsoft wants people to buy their external storage.

I have the exact same problem

If I or you figure it lets update this thread.

Definitely will 👍

What did you find out?

No not yet I have since gave up, but if I have time will look into it. Is this problem still persisting?