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External HDD not working when used as internal drive - 2011 iMac 27"

So this is the issue: I've got a 3tb StoreJet external drive and decided to install it inside my iMac after I upgraded the original drive to a SSD.

The SSD works and the DVD works but the HDD is not being recognised by Sierra even though it works fine when plugged with the external case - it does show up on Disk Utility but it asks me to reformat even though it works flawless when plugged as external.

I moved it back and forth a couple times to me sure and it does work when plugged in the external case but does not work when plugged inside the iMac.

Also the CPU fan went crazy and won't back down no matter what - tried PRAM reset, tried smcFanControl, tried SSDFanControl, nothing seems to quiet it down.

When I turn the iMac off then on again it stays quiet for about a minute, then it kicks in again full blast.

Checked all sensors, it's all plugged in correctly and the readings seems correct as well but the CPU fan will keep on going at 3800rpm even though the CPU reads only 30ºC.

Don't know if it's related to the HDD issue or if I f*cked up something on the logic board with all this back-and-forth due to the HDD issue :(

Any ideas?

Update (08/26/2017)

After booting from Recovery Mode the CPU fan seems to be under control even though I've done nothing, just boot and restart.

The HDD still shows as 'partition damaged' if installed as internal drive, same thing if I use a different USB external bay but when plugged into the Transcend StoreJet USB external bay it works flawlessly.

Linux does the same thing which makes me wonder: is it a Unix thing that formats external drives as external only? This is VERY frustrating, I can't believe I'll have to buy another drive to use as internal then transfer all my files from this external one… ¬¬

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Bought another HDD, installed inside my iMac, reformatted and now everything works fine.

Next step was to transfer everything back and done.

Still doesn't answer why a external drive wouldn't work when installed internal on a Mac or why the other way around - getting a internal to use as external - works fine. Wish I knew…

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You have a couple of problems here. While Macs are fine booting externally from a disk with any format, internally they MUST be formatted as GUID Mac OS X extended. Your external is probably formatted MS DOS or FAT.

Second, when replacing the internal drives, the heat sensors on the Apple priority drives prevent just dropping in any off-the -shelf drive. You will need to get an OWC 2011 thermal sensor to prevent run away fan issues. Here's what you need: https://eshop.macsales.com/item/OWC/DIDI...

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Not really... It was formatted as GUID using Disk Utility under Sierra in this very same iMac.

Talking to a friend who serviced thousands of Macs over the years it seems OSX is fine with internal HDDs being used as externals but not the other way around - and that's where I think it might be a Unix thing since booting from a live Linux doesn't fix the issue.

It is very frustrating and I still don't know why it works like that but then again I should've known it wouldn't work - I remember this is the second time this happened to me, third if I count the time I tried to install my external inside an Apple AirCapsule.

Anyway, I just wish I understood Unix well enough to know why things are like that. There must be a reason - perhaps to protect the data against corruption?

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