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Asus STRIX Z270e Motherboard only showing one SSD after installing MX300 SATA M.2

MB: Asus STRIX Z270e Gaming

I currently have a Samsung 850 EVO SSD connected by a SATA cable into the SATA6G_1 slot on my motherboard. I have my Windows 10 boot installed onto this SSD and everything has been working fine. However, recently I tried to install a 1TB Crucial MX300 SATA M.2 into the M.2_1 slot on my motherboard (this is the only slot that supports SATA, the other M.2 slot can only do PCIE). When I put the M.2 SSD in, I boot up the PC and it booted into the BIOS since for whatever reason it was only recognizing the new M.2 SSD I just put in. No where on the bios could I see my existing Samsung SSD, so I was just stuck in the BIOS since I could not select to boot off the the Samsung SSD. When I take the M.2 out, then Windows 10 boots up just fine off of my Samsung SSD as usual.

I am not sure what is causing my motherboard or BIOS not to recognize the Samsung SSD after I put the MX300 M.2 SSD in. Is there something I am missing here? Is my motherboard making the SATA port that my Samsung SSD is connected to inactive after I put an M.2 in? Help would be greatly appreciated!!
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  1. Best answer
    connect the 850 EVO to other port.
    when you install M.2 SSD it disables first SATA port.
    you can find it in user manual.
  2. n0ns3ns3 said:
    connect the 850 EVO to other port.
    when you install M.2 SSD it disables first SATA port.
    you can find it in user manual.


    Ah that was it! I was thinking that as well but I don't understand why that would of been the case, I couldn't locate the reasoning in the manual. Thank you!
  3. The reason is simple, the MX300 is using SATA protocol in M.2 form factor. and thus have to be connected to a SATA controller which is also connected to the SATA ports. so when you use SATA M.2 SSD it simply routs one of the SATA ports to the M.2 port.
    At least that's how I understand it.
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