Don't think there is a way unless one get into developer mode to modify the ADB. However, one can always use this kind of USB hub that has separate switch that can turn off the power to the USB device. Hope it will help
On my Pixel3 there is a developer option "default USB configuration" and available options are:
File Transfer
USB Tethering
Midi
PTP
No data transfer
Maybe there's a similar setting for AndroidTV Pie.
I was also facing the same issue but after trying following I got the solution for this.
You can set your USB drive as storage device for TV, in that case you will have to format your USB drive once. After you switch off and switch on the TV, you will no longer see the annoying popup in the start up.
Hope this helps.
It keeps saying new USB
I want to keep a USB plugged in as much as possible but it's annoying to have that popup everytime I boot it up
Details:
MiTV-AXSO0 32 inch Android 9 Kernel version: 4.9.113