Can't see External Hard drive. Help (with pics)

Did you perhaps format the external drive in APFS format when

you upgraded to High Sierra on the old iMac?

If you did, the older Sierra macOS will not see any volumes no matter what

you do as APFS incompatible with anything other than High Sierra.

I would upgrade your new Mac to High Sierra and then see if the volumes on the

external drive are recognized and go from there.

Actually, what I would do, download the High Sierra installer, make a USB

installer from it, boot the that, wipe the internal drive and do a clean install.

If you are simply trying to migrate the data from your old computer to your new iMac then connect the clone External Hard Disk from the old computer to the new one via a USB cable. Then launch Migration Assistant, you will find MA in Applications - Utilities - Migration Assistant. You can use Spotlight to easily find it too or you can use your 90 days of complimentary AppleCare telephone support and call for help.

Complimentary AppleCare Support

If you bought the computer used, all bets are off. We have no idea what you got. My suggestion is to take it to your local Apple Store for a free diagnosis.

When you got the computer did the previous owner do 100% of What to do before you sell or give away your Mac - Apple Support if not then at best you can do an erase and install and start over.

It looks like that external drive is either un-initialized, a weird format, damaged, or owned by an Admin that is not the same as your UseriD.

can you use Terminal and type:

diskutil list

then paste the output back here?

I think that Sierra can recognize that the drive is an APFS drive, but may not be able to do a lot beyond that.

Apple has typically moved recognition into the previous major version, then turned the features fully on in the next major version.

It is possible that Western Digital does not support booting from that drive. I have had a similar problem with a WD drive and I contacted the manufacturer. They told me that the drive was not bootable. The drive would mount fine after the system was booted, but there was nothing I could do to get it to appear in the boot menu.

Even if the drive doesn't show as a bootable drive, you should still be able to access it after you boot. I may have misunderstood your problem. I thought you were trying to boot from it.

If you can't access it at all, there could be another problem. Make sure you aren't using a USB hub that doesn't give the drive enough power to operate. That is another problem I've run into. My USB BluRay drive doesn't work on my new iMac when plugged directly into it, because it can't get enough power. It works fine on my Mac Pro. I just ordered a powered USB3 hub to see if that will fix the power problem.