According to Copying an operating system image to an SD card using Mac OS steps are

Info

diskutil list

(snip)
/dev/disk4 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *32.0 GB    disk4
   1:                 DOS_FAT_32 NO NAME                 32.0 GB    disk4s1
(snip)

Unmount

Take disk identifier and unmount disk