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  • When I install my RAM in slots 1 and 3, the computer won't POST.
  • When I install my RAM in slots 2 and 4, it starts, and works fine.
  • When I install my RAM in slots 1 and 4, it starts, and complains that I don't have the RAM in the right slots to use both channels.

I'm looking to upgrade my RAM. Should I be concerned that my motherboard is bad, since it won't POST with certain RAM configurations? The motherboard manual recommends using slots 2 and 4 if you're using two sticks. I was under the impression that this was just a recommendation, not a requirement. Is that correct?

My motherboard is a MSI Z370-a pro

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    You've still options to test… 1&2, 3&4 would potentially confirm an issue with slot 3. Check, if it boots, that you can see all the RAM in both cases. – Tetsujin Feb 26 at 19:14
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    It's also good to test individual slots alone, in case the specific slot is having trouble. I.E. put RAM only in Slot 3 and try it. I suspect all of the slots will work alone, due to SamForbis' answer below. Knowing Slot 3 is trouble will inform you as to whether to try upgrading to all 4 slots in the future. – Christopher Hostage Feb 26 at 19:36
  • Always read the manual for things like this if you want the optimal configuration for your motherboard. – Thorbjørn Ravn Andersen Feb 29 at 13:26
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Looking at the manual for your motherboard from MSI's website, it says the following:

Always insert memory modules in the DIMMA2 slot first.

So no, it's not just a recommendation. It's a requirement, although the wording on the diagram in the manual is misleading. You must always install the first module in DIMMA2, but it's up to you how to install after that first one.

You certainly want to follow the manual's recommendation when it comes to which slots to use, as installing the memory incorrectly could limit memory throughput significantly (you want to be able to leverage dual-channel memory whenever possible).

The slots on your board are labeled DIMMA1, DIMMA2, DIMMB1, DIMMB2. DIMMs A2 and B2 should be filled first to leverage dual-channel memory. If you put one in A2 and another in A1 or B1, the machine might boot, but it will not run in dual-channel mode. Follow this diagram for installing memory (also from the manual):

Recommended memory setups

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    Nice bit of research! if you want to see complicated RAM slot mapping, see the old 2008 Mac Pro - this from OWC - eshop.macsales.com/tech_center/manuals/Memory/macpro.pdf Not exactly intuitive ;-)) – Tetsujin Feb 26 at 19:43
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    @Tetsujin Leave it to Apple to complicate something like that even more than it already is! You'd think that with as far as we've come in terms of technology, something like this would be a lot simpler... – Sam Forbis Feb 26 at 19:51
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    RAM installation has had instructions / configurations like this for decades, going back to chip locations and SIMM modules. – Mick Feb 27 at 8:21
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    As for why they want to to install DIMMA2 first as opposed to DIMMA1, this is for electrical reasons. A circuit board trace with nothing at the end of it is a "stub" and it creates reflections. After all, electrical signals are carried by electromagnetic waves, and they can bounce off of stuff just like light can bounce off of a window. Therefore, you have to populate the memory module at the end of the trace first to minimize these reflections and ensure proper operation. – alex.forencich Feb 27 at 20:51
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    tl;dr RTFM. Seriously, people. – Ian Kemp Feb 28 at 7:57

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