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Oh, that's neat. So much flexibility with these Mac Pros. Wish I had gotten one earlier.
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If I can successfully install the OS on the WD drive, can I then try putting it back in the iMac and see if it works there?
Yep, that's the idea.
If I remember correctly, the one iMac that don't accept drives with Mac OS installed from a Mac Pro 5,1 is the 2009 model with Nvidia 9400 chipset, every other one works nicely.
 
Yep, that's the idea.
If I remember correctly, the one iMac that don't accept drives with Mac OS installed from a Mac Pro 5,1 is the 2009 model with Nvidia 9400 chipset, every other one works nicely.

Appears the OS installation was a success ;)
[doublepost=1534897680][/doublepost]Just booted up the iMac and got the dreaded flashing folder/question mark warning. Gonna mess around in recovery mode and see if I can get it going. So far, I am just getting the spinning globe.
 
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By the way, I do have the original iMac 2010 discs that came with it. I suppose of all else fails, I could try that.
 
By the way, I do have the original iMac 2010 discs that came with it. I suppose of all else fails, I could try that.
Do you have another drive, an old working one? I'm starting to suspect that your iMac firmware is so old that it can't understand AF/4K format.
 
Do you have another drive, an old working one? I'm starting to suspect that your iMac firmware is so old that it can't understand AF/4K format.

Just the original drive from the iMac, which still works but has SMART errors that can't be fixed. I discovered the SMART error after trying to upgrade the OS to El Capitan. After the error, it wouldn't let me upgrade.

Right now, I am in the process of re-installing High Sierra through recovery mode.
 
Just the original drive from the iMac, which still works but has SMART errors that can't be fixed.
SMART errors impede the firmware upgrade. Any old working drive will work to upgrade the firmware, you can use even a 2.5" one.
 
I only have a second WD 2TB drive that is new. Right now, it's stuck in the High Sierra re-installation where it asks you to select the disk on which you want to install. Shows no disk.
 
Put that drive on your iMac plus your createinstallmedia 10.13.6 pendrive. Go to your iMac, boot from the createinstallmedia and install 10.13.6 on your USB external drive. It will be a lot longer install, but will work.
 
Put that drive on your iMac plus your createinstallmedia 10.13.6 pendrive. Go to your iMac, boot from the createinstallmedia and install 10.13.6 on your USB external drive. It will be a lot longer install, but will work.

Good idea.
[doublepost=1534904530][/doublepost]When I go to install it, says the external drive doesn't use the Guid Partition Table scheme. Tried erasing, no luck. Must be a job for Terminal.

Think I got it GUID partitioned. Will go ahead with resin tall.
[doublepost=1534905405][/doublepost]New error: "You may not install on this drive because computer is missing a firmware partition."
[doublepost=1534905537][/doublepost]I'm thinking maybe reinstall from original iMac discs to the external drive, then updating from there, yes?

UPDATE: reinstalling Snow Leopard onto external drive then will update to El Capitan or whatever else is available.
 
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UPDATE: I've managed to upgrade my iMac to OS Sierra using the external drive; however, the iMac still won't recognize the Western Digital hard disk. In the process of getting High Sierra installed...after a few Sierra updaters.
 
What's the current firmware? (SystemProfiler/HardwareOverview/BootROM)

I just compared these specs with the iMac prior to the upgrades. Not a difference.
 

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You need to install High Sierra, the newer ones are installed by 10.13.6.

Yeah tried that. I just get an error saying "you may not install to this volume because the computer is missing a firmware partition".
 
Yeah tried that. I just get an error saying "you may not install to this volume because the computer is missing a firmware partition".

Maybe it will be easier if you install a old/small hard disk or a small SSD internally. I'm suspecting that your current BootROM don't support 4K/AF that big hard disks uses today.
 
It's strange. I took the WD drive out of my iMac and put it back in the Mac Pro. It had OS Sierra installed on it even though I only supposedly did that on the external drive while attached to the iMac. Oh well.
 
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