First check your equipment i.e. use different cables, imaging station and write-blockers. Also, update FTKi. If you are still getting the same errors try another methodology i.e. image the hard drives in the iMac (or any pc chassis) using a suitable forensic imaging boot disk (Paladin, DEFT, Helix, Macquisition). If you are still receiving errors it is likely you have some failing disks and may need to seek data recovery experts.
Rgds
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