How do I list the plugged memory sticks on Mac OS
You can use the native OS X diskutil command as shown below:

$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            499.2 GB   disk0s2
   3:                 Apple_Boot Recovery HD             654.6 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *8.1 GB     disk1
   1:                 DOS_FAT_32 LACI                    8.1 GB     disk1s1


If you're doing this from the command line the following command would list the Volumes currently mounted and their mount point in the filesystem:

$ df | grep ^\/dev\/disk
/dev/disk0s2                      487542528 377409836 109876692  78% /
/dev/disk1s1                        7879680   1128124   6751556  15% /Volumes/LACI


If you're just interested in the physical disks:

$ df | grep -i ^\/dev\/disk | awk {'print $1'}
/dev/disk0s2
/dev/disk1s1


Or the mount points:

$ df | grep -i ^\/dev\/disk | awk {'print $6'} 
/
/Volumes/LACI