Any chance to recover data from damaged HDD partition?

Was Filevault enabled on this iMac? If so, then within Disk Utility select the "disk4s2" item and click the "Mount" button which should prompt you to unlock Filevault.

Or did the iMac have a Fusion Drive? A Fusion Drive is when an iMac has both a hard drive and an SSD which are combined together. If this iMac had a Fusion Drive, then you need to have the SSD as well to access the data. However, now that the hard drive has been removed it may have marked the Fusion Drive as split so it may no longer be possible to access the Fusion Drive even if the hard drive is placed back into the iMac.

More than likely the hard drive itself is worn out or even failing which may be the real issue here if no Fusion Drive is involved. In that case the more you attempt to access the failing drive, the more likely you are making the failure worse where even an expensive professional data recovery service such as Drive Savers will be unable to recover any data. You can try checking the health of the hard drive by using DriveDx, but you will need to install a special USB driver to attempt to access the health information of the external drive. Even with the special USB driver, there is no guarantee that the USB adapter, drive dock, or enclosure will allow the necessary communication to the health information on the external drive. If you can access the drive's health information, then post the complete DriveDx health report here using the "Addtional Text" icon which looks like a piece of paper.

ari-1979 wrote:

Thanks! I really appreciate your feedback. Would it be a good idea to get the SSD out and bring both drives to an unofficial Apple repair store (not a professional data recovery company) to get the data transferred?

Theoretically yes, but I honestly don't know if the Fusion Drive will be intact especially now that an attempt was made to access it without the SSD portion....so it may now appear split and I'm not aware of any method to salvage a split Fusion Drive where the data is retained. The problem with having both drives connected externally is that if they don't go ready together, then macOS may see the Fusion Drive as split. It probably can't hurt at this point.

So far I did not do anything to the HDD other than to scan it with DiskDrill. I did not recover any files after I scanned the drive.

You will probably need to do a more thorough & deep scan. If Disk Drill does not provide that option, then try another data recovery app like Data Rescue (I know this one had that option years ago....haven't used this app in many years).

What version of OSX on your Mac & the bad drive?

If not APFS format Your best bet is DiskWarrior...

http://www.alsoft.com/DiskWarrior/

Did the iMac even power on? If so, then you can try putting the iMac into Target Disk Mode and connect it to another Mac to try to access the data on the Fusion Drive assuming the Fusion Drive was not broken when you removed the hard drive to try to access the data. And assuming the hard drive is even healthy. Very little of the iMac must be functional to access Target Disk Mode.

There is a possibility you may be able to use a data recovery app to try to recover some of the data from just the hard drive portion if the Fusion Drive is considered split, but it may not be easy. Sometimes when recovering data from a drive like this you may not get any file names with any of the recovered files which means you may end up with millions of files which could potentially hold the data you want and perhaps even multiple copies from temp & cache files. It doesn't hurt to try though. Keep in mind if the hard drive is failing, then the data recovery app won't be able to handle the physical errors produced by a failing hard drive. I've never personally tried to recover files from a split Fusion Drive before.