I found a workaround: In Router setup: Basic>USB Settings>Folder Sharing> Turn off "Share All". Specify some folders to share. Now you don't have sda or sdb drive/folder in the first place. Shorter...

@Kevin_Z Updated to latest (firmware version 1.1.3 Build 20210519 rel. 50598(4555), but the router still changing USB drive letters. The most annoying thing is that changes occur while using, like...

@Kevin_Z 1.1.0 Build 20201120 rel.50406(4555)

I have the same issue (Archer C7 v5.0). Letters changing about every hour, in the middle of watching some movie. No solution found yet.