it seems that whatever step is trying to mount the USB device has to wait until initialization of the USB driver for that device is done.
It doesn't know it's trying to mount a USB device. All it knows is block devices, and the one it was told to mount doesn't exist.
While it's undesirable to have the USB driver hold up the boot, because the boot may not depend upon any device on a given hub, it might make sense to have a driver delay an open until the full initialization has taken place, to see if the device shows up.
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