The right stuff made it easy.

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The factory Seagate 3tb hd died.

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The iMac opening tool was perfect and the adhesive strips are top quality. The little blue tabs made it very easy to strip of the backing. They are also cut to fit perfectly, unlike the generic strips some companies sell.

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I used an HGST enterprise 4tb drive. I've had great luck with the HGST drives. This machine has the Fusion drive. Disk Utility recognized the new drive and automatically reconfigured the Fusion to the new drive. No command line required.

Also used the OWC cable to enable the iMac to get drive temp to properly control fan speed.