The device name of USB devices depends on the number of partitions.
For USB devices with one partition:
/usb2msd16a
For USB devices with two partitions:
First partition: /usb2msd16a
Second partition: /usb2msd16b
For USB devices with three partitions:
First partition: /usb2msd16a
Second partition: /usb2msd16b
Third partition: /usb2msd16c
USB devices with even more partitions follow this pattern.