$ diskutil list
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *320.1 GB disk0
1: EFI 209.7 MB disk0s1
2: Apple_HFS Hard Disk 319.2 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
/dev/disk1
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *8.3 GB disk1
1: DOS_FAT_32 DISK_IMG 8.3 GB disk1s1
/dev/disk2
#: TYPE NAME SIZE IDENTIFIER
0: CentOS-6.2-x86_6 *731.9 MB disk2
$ diskutil unmountDisk /dev/disk1
Unmount of all volumes on disk1 was successful
$ dd if=/dev/disk2 of=/dev/disk1 bs=1m
dd: /dev/disk2: Resource busy
$ diskutil unmountDisk /dev/disk2
Unmount of all volumes on disk2 was successful
$ dd if=/dev/disk2 of=/dev/disk1 bs=1m
698+0 records in
698+0 records out
731906048 bytes transferred in 206.409147 secs (3545899 bytes/sec)