I have upgraded my 2Tb Seagate TC to a 4Tb Green WD. Works well, and not that tricky. Hardpart was twofold: 1 Getting the thing apart is not that easy.......get yourself some non scratching plastic spludgers that are STRONG. I broke off the end of one of mine getting the bottom off. 2 The foam in the bottom of the TC is designed to take SEAGATE drives. The metal folding is slightly different in the SEAGATE. To get the WD to fit snugly I had to take a sharp knife (I used a scalpel) and slice off the bits of foam that are sticking in the way of the bottom of the WD drive. Once I did that it fitted in the case as a factory fit. I am actually thinking of buying a 6Tb drive and upgrade it again. Cheers to all. PS the price from Apple of the 3Tb drive is crazy. Get yourself a refurbed 2Tb and upgrade it to 4 or 6 Tb.
Regarding upgrading a 2011 27' iMac for Handoff: I have used the BCM94360CD combo card to successfully upgrade my 2009 Mac Pro. Handoff works well. I have a spare one of these combo cards and mini pcie adapter. I have also ordered a BTooth antenna extension cable . What I do not know is if the combo card needs to get USB power from the old BTooth slot (after I removed the old card) via a separate connector cable. Anyone know this small but important fact. I did not want to pull the machine apart (again) until I knew if I need to source a mini pcie with USB power connectors and the cable to supply the power.