I wanted to upgrade the memory on an iMac (Retina 5K, 27-inch, Late 2015), which had 2x4GB chips. I bought 4x4GB chips (I only needed 2, but bought so many to use in other machines).
None of the 4 chips worked - as in, swapping them out one by one, making sure they were well seated, the 2 original chips worked fine, but with the 4 new ones, the machine would not boot at all.
The original chips, made by "SK hynix", have a sticker that says:
4GB 1Rx8 PC3L - 14900S - 13 - 13 - B4
HMT451S6BFR8A - RD NA AA 1638
The new ones, made by "Komputer Bay", have a sticker that says:
4GB PC3-14900 DDR3
2RX8 NON ECC 1.35V
The seller (on Amazon) states these new chips are made for this model of iMac. Is the seller wrong? Or could I have been given 4 defective chips?
Here is the link to the Amazon page, which states "..for Apple iMac 275K (Late 2015)". Thanks to @monomeeth for that - it is possible I was sent the incorrect type.