I added a depot writer to a request depot and a signal to store up to 400 copper plates ([copper-plate] = 400). I placed a supply depot next to it, and put 200 plates in it, and nothing happens. The drone never moves. If I add another 200 plates to the supply depo, the drone starts working and brings all 400 to the request depot.

Without a writer attached, the drone will always brings the plates, no matter how many are in the supply depot.

Actually, this appears more to be a problem with supply depot needing to have the maximum amount rather than stack size. I have updated the post (but can't change the title).

But when I tried with limit 600 and 400 plates, it worked. So maybe it is related to stack size (I'm using Krastorio 2 so the stack size is 200, and the drones are upgraded to carry two stacks).

Can you provide a save game demonstrating the issue?

I play with a lot of mods so maybe it's a compatibility issue, but anyway, here's the save file: http://cdn.modo.lv/TransportDrones.zip

I have set the request depo to request 600 iron plates and a supply depot with 250 right next to it, and nothing is getting transferred. If I add another 200 plates to the supply depot, the drone starts moving immediately.