installing memory iMac 27" 2017 8GB to 64GB

"I purchased 4x16Gb of A-Tech from Amazon."

Return for a refund, the RAM you got will not work. As you have been advised iMacs are EXTREMELY fussy about the RAM that can be installed. Buy your RAM from macsales.com.

My spin: "GOOD RAM WORKS BAD RAM DOESN'T"

good RAM should plug and play -- one should not have to play around with them -- unless troubleshooting to find the problem card(s)

if all four cards match, you don't have to worry about placing them in their proper slots -- just seat them securely

if you have mismatched pairs, each pair needs to match and be placed in the proper slots

I run 128GB (OWC ) in my 2019 5K

and 64GB (OWC) in my 2019 4K

I always max RAM in my personal machines

I've seen minimum RAM slow iMacs down, but I think HDD (spinning hard drives) and small SSDs are much worse for the spinning color wheel

macOS (and apps like Photoshop) generate "swap files" virtual memory, Scratch Disks temporary files they write to the HD/SSD -- more RAM will write/read those files a lot faster in RAM than if they have to write to and read from much slower SSD/HDD

that's why you don't want to cheap out on RAM -- add as much RAM as you can afford or feel confident, the internal drives are a lot harder to upgrade -- upgrading RAM on 2019 21.5" is a complete disassembly...

+1

What RAM vendor did you purchase the 4x16=64GB upgrade from..?

Those 27" iMac's are notorious for not booting a full load of after-market modules, if the spec's are even slightly off.

Return that RAM and buy a 64GB upgrade kit directly from OWC.

https://eshop.macsales.com/shop/memory/imac-2017-27-inch

You could replace the two new ones that work with the other two new ones that still work, and see if they work also. But, the best idea is to purchase OWC RAM.

ilovemyipodnmusic22 wrote:

I purchased 4x16Gb of A-Tech from Amazon. I get a black screen with all 4 16GB in the slots. But if I put the original 2 back in the slots they were in with 2 of the 16GB my iMac turns on.

Macs can be very picky about RAM.

There used to be two "go to" vendors who sold RAM guaranteed to be compatible with particular Macs: Crucal and Other World Computing. Crucial got out of that business and OWC is the only one left standing.

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Please download and run Etrecheck.  The free version is sufficient.

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and after clicking on the Reply button use the Additional Text button to paste the report in your reply.

Check Etrecheck's Privacy settings and make sure the checkbox for full disk access before running:

Then we can examine the report and see if we can determine the cause of the problem.

If you mean you bought 4 16GB cards and your iMac won't boot with all 4 installed, one or two of the cards may be bad or out of spec. What brand are the cards?

Why are you wanting to up the RAM to 64 GB? Is it to speed up your iMac?

If that's the case it's not RAM that speeds up a Mac, it's the speed of the boot drive. If you have a Fusion drive in your iMac it may have the 5400 rpm HD and that could be the bottleneck. It might be failing or too full.

Let us know the reason for the increased RAM and we can go from there.

2x4+2x16=40.