Do you keep the external USB drives plugged into the same USB ports and do
they all boot up with the machine every time?
For example a card reader that is built into a desktop and connected to a
motherboard USB port passively activates every time the system is booted and
the card slots always get assigned the same drive letters.
Not so for external USB drives which may not always be turned on when the
system boots or may be plugged into different USB ports.