How do I upgrade memory in vacant 2 slots at rear of iMac Retina 27 (Late 2014)

There's no problem adding pairs of 4 GB or 8 GB modules if they are purchased from OWC (MacSales.com) or Crucial.com. These two vendors guarantee that their modules will work with the models they are intended for. We get a lot of inquiries of economy modules, which had the right specs, that wouldn't work. You get what you pay for.

Be aware that additional memory will not speed up boot time or application launch time. That's achieved thru a faster boot drive, like a replacement SSD or external SSD.

You can add two compatible 4GB or two comparable 8GB modules.

Watch the Install Video @ https://eshop.macsales.com/installvideos/imac_27in5k_14late_mem/

Then buy the 8GB (4GBx2) or 16GB (8GBx2) upgrade @ https://eshop.macsales.com/shop/memory/iMac/Retina-5K/DDR3L

marietheauthor wrote:

There are 4 memory slots at back of my beloved iMac Retina 27 purchased new in 2015. 2 of these slots are empty and require a component to make them useful_ What are the prospects/consequences of installing 2 1600 MHz DDR3 memory modules?

Empty RAM slots are your friends.

Apple chose to ship the 8 GB of RAM in your Mac as (2 x 4 GB). Installing another (2 x 8 GB) of RAM would give your machine a total of 24 GB of RAM. If Apple had shipped the RAM as (4 x 2 GB), you'd have to remove 4 GB worth of factory-installed RAM to put in (2 x 8 GB), so a two-stick upgrade would only get you to 20 GB.

If your Mac only had two RAM slots, its maximum capacity would have been cut in half, from 32 GB to 16 GB.

If these slots are able to be used and do not impinge on the computer's performance why didn't Apple put these components in when the computer was being assembled?

When you bought your iMac, would you have wanted to pay for 32 GB of RAM at Apple prices?

You're welcome.

You're welcome.