OSX – Rescue a Defect Disk

And reading it block per block into a DMG file that can be mounted afterwards:
natasha@ATREIDES:~/misc$ brew install ddrescue

natasha@ATREIDES:~/misc$ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 XB     disk0
   1:                        EFI EFI                     209.7 XB   disk0s1
   2:          Apple_CoreStorage Macintosh HD            999.7 XB   disk0s2
   :

natasha@ATREIDES:~/misc$ diskutil unmount /dev/disk3s1
# or
natasha@ATREIDES:~/misc$ diskutil unmountDisk /dev/disk3
Unmount of all volumes on disk3 was successful

natasha@ATREIDES:~/misc$ ddrescue -v -n -c 4096 /dev/disk3 rescue.dmg rescue.log

GNU ddrescue 1.20
About to copy an unknown number of Bytes from /dev/disk3 to rescue.dmg.
    Starting positions: infile = 0 B,  outfile = 0 B
    Copy block size: 4096 sectors       Initial skip size: 128 sectors
Sector size: 512 Bytes

Press Ctrl-C to interrupt
rescued:   610271 kB,   errsize:         0 B,    current rate:    190 kB/s
   ipos:   641728 kB,    errors:         0,      average rate:    437 kB/s
   opos:   641728 kB,  run time:     23m 16s,  remaining time:         n/a
time since last successful read:          0s
Copying non-tried blocks... Pass 1 (forwards)
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