windows 10 update assistant doesn't see my usb drive

The assistant is picky, it wants the stick to show up as a proper local disk and some sticks present as removable media which it ignores. Try a different port on the back of the machine, not a hub, and if you have an external hard drive use that instead. The better fix is making the space on C: so it doesn't need a drive at all, Storage settings > Temporary files, uninstall a couple of big apps, empty Downloads. It needs around 10 GB. Or skip the assistant and run the Media Creation Tool, that handles space differently.