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