"So any usb drive I connect stays in the same letter?"
No this is not possible for any usb drive you may encounter, Windows automatically assigns drive letters unless it is a device it has seen before and you have assigned a drive letter to it previously using Disk Management. Sometimes this does not work either if that drive letter is being used by another device (for what ever reason) when you plug in the one you assigned the same drive letter.
By automatically I mean Windows chooses the next available drive letter down the alphabet string that is not being used.
No way to force it assign M every time on a usb device it has never seen before. This would be a bad idea if another device is already using M when you plug in another.
All this being said, you can manually assign M (in disk management) to any usb storage device, once you manually assign it M, Windows will remember that particular device and assign M automatically if no other device is using that letter when you plug it in.
1It looks like the proposed duplicate covers the opposite objective, i.e., ensuring each device retains its uniquely assigned different drive letter. This question seeks to have any/all devices assigned the same designated drive letter if plugged into the USB port (port-centric assignment vs. device-centric assignment). – fixer1234 – 2016-05-01T04:55:29.687