$ diskutil list
...
/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                            FREE_SOLO              *4.6 GB     disk2

$ diskutil mount readOnly disk2
Volume on disk2 failed to mount; if it has a partitioning scheme, use "diskutil mountDisk"
If the volume is damaged, try the "readOnly" option