To mount a disk (usually a file system that you usually go and click on using nautilus/caja (file browsers)) from the command line, use the following,
udisksctl mount -b /dev/disk/by-label/
Listing the content of
/dev/disk/by-label/ will give you the names of the mountable disks/drives. Usually flash drives are mounted automatically so they do not have to be mounted. Unless you unmount it and want to mount it again. Use
man udisksctl for more information. Also, I think it is udisks in some distributions.