Well, yes, if you tell a system to mount a physical device that isn't actually
where the system expects to find it, you can expect to get an error message.
Having never tried to auto-mount a USB resource in Linux I have no idea what
form that message will take.

It seems to me you have to options:
1) make sure the USB drive is always there and plugged into the same USB port.
2) manually launch a small script after boot to find the drive and mount it.