Remove disk image from the disk list
hdiutil will show information about all mounted DMGs:

hwd@hwds-iMac:~$ hdiutil info  
framework       : 415
driver          : 10.11v415
================================================
image-path      : /Users/hwd/Downloads/mountedImageOne.dmg
image-alias     : /Users/hwd/Downloads/mountedImageOne.dmg
shadow-path     : &lt;none&gt;
icon-path       : /System/Library/PrivateFrameworks/DiskImages.framework    /Resources/CDiskImage.icns
image-type      : UDIF, nur Lesen, komprimiert (bzip2)
system-image    : false
blockcount      : 22528
blocksize       : 512
writeable       : false
autodiskmount   : TRUE
removable       : TRUE
image-encrypted : false
mounting user   : hwd
mounting mode   : &lt;unknown&gt;
process ID      : 5376
/dev/disk2      Apple_partition_scheme  
/dev/disk2s1    Apple_partition_map 
/dev/disk2s2    Apple_HFS   /Volumes/mountedImageOne
================================================
image-path      : /Users/hwd/Downloads/mountedImageTwo.dmg
image-alias     : /Users/hwd/Downloads/mountedImageTwo.dmg
shadow-path     : &lt;none&gt;
icon-path       : /System/Library/PrivateFrameworks/DiskImages.framework/Resources/CDiskImage.icns
image-type      : UDIF, nur Lesen, komprimiert (zlib)
system-image    : false
blockcount      : 20480
blocksize       : 512
writeable       : false
autodiskmount   : TRUE
removable       : TRUE
image-encrypted : false
mounting user   : hwd
mounting mode   : &lt;unknown&gt;
process ID      : 5416
/dev/disk3      GUID_partition_scheme   
/dev/disk3s1    48465300-0000-AAAA-AAAA-000000000000    /Volumes/mountedImageTwo