Installing an 8TB HD in an iMac?

Looks like the upper limit of OS X is just under 8 exabytes, 8 million terabytes...

(9,223,372,034,707,292,160 bytes)

Mac OS X: Mac OS Extended format (HFS Plus) volume and file limits - Apple Support

You will need a temp sensor, original drive had it built in...

https://eshop.macsales.com/item/OWC/DIDIMACHDD11/

(This can be found on Amazon too)

https://www.amazon.com/OWC-Digital-Thermal-Sensor-Upgrade/dp/B00J42AFUA/ref=sr_1 _2

Disassembly is a little tough,

https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+EMC+2428+Hard+Drive+Replacemen t/5954

At step 4, do NOT lift the glass to much, you only have about 4 inches MAX.

I slide a paperback book in, then disconnect the cables.

Get an external case, USB or FireWire,

if it supports 8 gigs (most do not), copy everything over before installing the new drive.

Otherwise, you will have to boot off the external, wich holds the old drive, and then copy....

Drive runs hot, put a fan on it, and some wood pencils underneath to help cool it, while copying before install.

https://www.amazon.com/ORICO-Toolfree-External-Enclosure-Support/dp/B00GAML7OK/r ef=sr_1_3?ie=UTF8&qid=1479626171&sr=8-3&…

Looking at this again, I would have to ask "Why do you need such a big hard drive?

My shopping list would be-

Western Digital Black, 2-5 terabyte...

http://www.newegg.com/Product/Product.aspx?item=N82E16822236971

8-16 gigs memory, you should have two slots free

https://eshop.macsales.com/shop/memory/iMac/2011/DDR3_21.5_27

along with the external case, and temp sensor...

(Sierra OS really likes at least 8 gigs)

For major external storage of items that you cannot afford to lose, a dual hard drive enclosure, running Raid 1 (a copy is made on both drives), would be my choice.

You will also need two identical hard drives...

https://eshop.macsales.com/item/OWC/MED3FR0GB/

16 gigs is fine.