Can Ram be tested if the Mac doesn't boot? Sounds like a funny joke, but please read on....

You did not mention your exact model Mac, so are your RAM slots arranged 2 side by side and 2 rows or the newer ones with 4 slots vertically? If the older model, the RAM should be matched side by side (so, 2 Apple sticks in one row and 2 other in second row). If the newer model, the slots should be matched in slot 1 + 3, and 2 + 4.

Having said that, your experience sounds a bit like what was going on a long time ago: the Macs refused to work with a mixed set of max. capacity RAM. They'd work with a full set of either Apple or OWC (or any third party) RAM, but if you mixed it, you had to stop short of going to the maximum (I had one of those Macs and it was a maximum of 16 GB (4 x 4 GB), but I had stop at 12 GB total and it worked).

So, during your testing, you may want to consider one test with 2 Apple and 1 OWC (or even one each). I had bookmarked two of those very old threads and can no longer find them (since the changes made to the forums here late last year), but I will keep trying and add the links here if I find them.

I had a similar problem with my 2019 iMac 27" 8 GB RAM. I bought two 16 GB from OWC which I added to the 8 GB. When I installed them, the Mac would not boot. What I realized after several trials is that you have to insert them very carefully by aligning them perfectly with their slots. What I mean is that the angle of insertion is very important. Another thing I found is that you really have to push them hard into their slots so that they are fully inserted.

I've always had good luck with OWC tech support in the past. They have always been easy to deal with and were always willing to exchange or return items without any hassles. I'm sure the COVID-19 situation is making things difficult for them especially if they are working from home. I'm sure they are getting a lot of support calls since people and organizations have been purchasing so many items to use for work at home or for system upgrades to bring old computers back to life.

If it takes time to detect a memory issue, then I understand it is time consuming and difficult. If you want to test the memory you have a couple of options to test the memory:

• Definitely try running the Apple Diagnostics.
• Run Memtest86
• Run "mprime" to stress the system (use the "Torture Test" mode of the utility). This is a great option for testing system stability. PC builders and overclockers have used this utility (aka Prime95) for years (since 1995) to test the stability of computers.

Your iMac should be using PC4-19,200 RAM (or 2,400 MHz) if you have the iMac 27" 5K (2017) model.

If the original Apple RAM pair works fine and the new OWC RAM pair works fine by themselves, then it is likely a compatibility issue with mixing RAM. If this is the case, then you don't need any help from OWC. If the new OWC RAM is not enough on its own, then you can purchase some more OWC RAM to get to the memory level you need. Sometimes you just cannot mix memory brands. For some systems some places even recommend buying all the RAM at one time so you get the same manufacturing batch of RAM since there are cases where mixing manufacturing batches of the same RAM can cause problems.

If you do what has been suggested here, then it will make things much easier once you get a hold of OWC tech support since all of the leg work has been done and you know whether any of the new memory modules are bad.

Also make sure the memory is fully seated into the memory slot. I've never upgraded or replaced memory on this model so I am not familiar with how easy or difficult it is to insert the memory. On some systems it can be a bit tricky to fully seat the RAM into the slot. If the Mac was dusty you may want to use compressed air to clean out the memory slots (just make sure to hold the can of compressed air upright so you don't damage the computer with the cold wet propellant).

The main thing would be to make sure the pairs of identical RAM are in the proper slots. One of the other contributors mentioned which slots should be used depending on the style of the slots. If the slots are labeled, then I think you need to install the identical pairs first into the DIMM 0 slots, then the second identical pair into the DIMM 1 slots. If you recall which slots held the original Apple memory, then those two slots should be used for the first pair of RAM modules.

That screen means that version of macOS on the drive isn't compatible with that Mac for some reason. It could be due to a memory issue or it could indicate a file system issue or corrupt OS.

https://support.apple.com/en-us/HT210901

Considering the history of this Mac a memory issue is a very likely possibility. You will need to run the Apple Diagnostics and Memtest86 to see if any errors are found. Unfortunately a passing test does not mean the hardware is good. It just means the tests were unable to identify a fault. The hardware can still have an intermittent issue.

Compressed air is much easier and safer. You are more likely to cause damage by inserting objects into the memory slots as you may damage the contacts (or cause more damage if the contacts are already bent). I am doubtful you will be able to clean the slots that way anyway since they are so narrow and a cloth is unlikely to fit inside. Try using a vacuum cleaner with a nozzle attachment to see if it will gather up the dust & dirt from the memory slots. This isn't quite as good as compressed air, but it may be sufficient at the very least so you can take some new pictures for us to review.

It would help to know the exact model of your Mac and exactly which memory slots you have installed RAM into and which modules are being used in each slot.

Yes, that can of compressed air will be fine.

Once you take new pictures of the clean slot with better focus and lighting we can better advise you. You may want to take a few pictures at different angles so we are able to get a better view since I know it is difficult to get good pictures. You can place the iMac face down on a flat surface (put a clean soft pad underneath so you don't scratch the glass) so that you may be able to get a better angle of the slots. Or you may need to have someone else hold the iMac at an angle which makes taking the pictures easier.

Good luck we are hoping for the best for you.

It wouldn't surprise me that a larger memory module may have issues when a smaller module does not. The larger module requires more power since there are usually twice as many chips. I don't know how the memory slots are wired, but perhaps some of the pins are not used on a smaller memory stick where they all may be used on a larger memory stick. This may show that the one memory slot that "works" may not be in perfect condition.

Some AASPs go over & beyond what Apple suggests so it wouldn't surprise me the AASP would have tried the OWC modules. An Apple Store on the other hand would be less likely to deal with third party items.

If you don't have a backup, then you can attempt to access the data on the internal drive by putting the iMac into Target Disk Mode and connecting it to another Mac.

If a contact in one of the memory slots is bent, then it may be shorting out against another pin which could cause your "good" memory & slot to have problems. Without being able to see the memory slots ourselves since you cleaned out the dust we cannot be sure. Even with good pictures we may not be able to tell for sure. I recommend you have Apple or an Apple Authorized Service Provider examine the iMac as this is unlikely to involve a simple fix.

It’s going to depend upon which year your iMac 27”, 5K, was manufactured, .

For instance, with the late 2014 model (see the technical specifications at https://support.apple.com/kb/SP707?viewlocale=en_US&locale=en_US), Apple only specifies compatibility with up to 32GB of RAM.

However. I have found that with the “march of technology”, one can, almost invariably, double that size.

The reason:

1. Apple only claims the RAM configurations they were able to successfully test at the time the machine originally came out.
2. Since that time, newer, higher density memory chips come available, with compatible specks.

One should go with one of the highly reputable memory supply companies, and ask them, specifically, what the maximum RAM they will guarantee will work in your particular system!

I did that with my Mid 2010 MacBook, that Apple rated at a maximum of 4GB RAM, and successfully upgraded it to 8GB RAM.

(However. With your “track record” of installing RAM, I highly recommend getting a professional to install it. Sorry. But true.)

I quite agree that OWC is reputable, Macs Are War Horses.

However, it is possible that an Apple Authorized Repair Shop is disallowed, by their contract, to deal with none-Apple sourced/authorized components.

So. One may need to search elsewhere for a professional to install the memory.

(Of course, if you feel sufficiently confident to do it yourself, I’m not going to stop you.)

By the way, what is the year of your iMac: the entire Model and Sub-Model shown in About this Mac?