New SSD won't appear on my Macbook Pro (mid-2014)

It would help to have some more details such as what SSD, adapter, and OS you are trying to use. If you installed an NVMe SSD, then the laptop needs to have had macOS 10.13+ installed at some point in the past while the original Apple SSD was installed in the laptop so that the laptop's firmware is updated to be compatible with an NVMe SSD. Plus you will only be able to use macOS 10.13+ on the SSD.

If you installed a standard M.2 SSD with an adapter instead of an OWC Aura SSD, then make sure to use a Sintech SSD adapter since users report compatibility issues when using any other SSD adapter. It is also possible your third party M.2 SSD is not compatible with the Apple laptop or perhaps the SSD or adapter is bad.

deaflg27 Said:

"New SSD won't appear on my Macbook Pro (mid-2014): How shall I resolve this?"

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Mount the Drive:

Boot into Recovery Mode (Hold Down: [Command + R] upon boot). Then, using Disk Utility, see if the Hard Drive, along with the boot disk are seen. If they are, then see if you need to click the “Mount’ button for it. Once mounted, reboot the Mac. If of no success, then click unmount button, and then the mount button. Try rebooting once more.