An imac retina 27" i5 (model 15.1) comes with 8GB (2X4GB) of RAM.
I am purchasing 2x8GB chipsets.
Will I cause a problem with my imac if I just add the 2x8GB sets yielding 24GB, or must I remove the original 8GB (2X4) first as apple suggests.
I just did the same thing the other day. Works fine. One thing I would note is that the memory that came preinstalled was in slots 1 and 3 so I just put the new memory in 2 and 4. It wouldn't startup! So I removed the memory and tried again. Put the original 2 x 4gb into slots 1 and 2 and the new 2 x 8gb into slots 3 and 4. All worked fine and have had no problems since.