When trying to mount a USB thumb drive in a Juniper EX3200 switch I received the following error:

root@:RE:0% mount -t msdos /dev/da1s1 /var/tmp/usb

mount_msdosfs: /dev/da1s1: Invalid argument

The file system was FAT and everything worked fine when mounted in my workstation.  I simply reformatted the FAT partition on my workstation and recopied the contents and it mounted correctly on the next attempt.