Crucial p3 ssd installation

I have got the Crucial P3 4tb to work on my 2015 Mackbook. I just mirrored my working ssd then installed it with the Sintech adapter it took a little longer on the start up, but everything is ok.

What does Crucial say about that compatibility of the P3 in a 2015 MacBook Pro running Monterey? It may simply not be compatible installed internally, and certainly any future update of macOS will fail as it does not find the original Apple SSD installed. If the external P3 solution is working, then use the path of least resistance.

Two things. What SSD adapter are you using for the internal installation? From my reading of these forums for years, the Sintech adapter seems to be the most compatible & reliable one. I have rarely seen anyone have issues with it.

Second, was macOS 10.13+ ever previously installed on this laptop? The laptop's firmware must have been updated to the new firmware which usually uses a different format that looks something like 192.0.0.0 instead of the old style which mixed alphabetic characters & numbers. If not, then you must first install macOS 10.13+ while the original Apple SSD is installed internally. Then and only then can you physically install the third party NVMe SSD. You will only be able to use macOS 10.13+ since older versions of macOS don't have the necessary NVMe driver for the new SSD.

If you are booting into Recovery Mode, sometimes a Mac may only boot into the older online macOS installer for the OS which originally shipped with the Mac from the factory which will be too older to work with the third party NVMe SSD.

Another possible issue with macOS 12.x Monterey.....if you did not install Monterey on this laptop while the original Apple OEM SSD is installed internally, then the macOS Monterey installer will refuse to install if the third party SSD is installed internally. Once Monterey has been installed while the original Apple SSD is installed internally, then the laptop's system firmware will already have been updated which means you can install Monterey at any time with the new third party SSD installed internally.

Keep in mind install is a critical keyword here since the laptop's firmware must be properly updated for these two situations when using a third party NVMe SSD internally.

I've seen a couple other posts a while back about people having issues with the Crucial P3 installed internally, but they also were not using the Sintech SSD adapter. I honestly don't know if the Crucial P3 is compatible even with the Sintech adapter since people don't always follow up with the results. However, the adapter is the source of 95% of people's problems using an M.2 SSD internally in these Macs.

Since Apple makes quality products and Crucial is known for quality products....it just makes since the unknown SSD adapter is probably the weakest link. If you use an adapter which is known to be fairly reliable like the Sintech adapter & you have issues, then it may mean the SSD is not compatible (or there is a problem with the SSD). This also assumes the old Apple SSD was working fine in this Mac and you are just upgrading to a larger SSD. If the old SSD wasn't working, then maybe there is an issue with the Logic Board (although the SSD is usually the weakest link everything else being equal).

Using another brand of SSD may work better with your unknown adapter, or it may just give you false hope with weird intermittent issues which you may not associate with the adapter. Hard to say since using any M.2 SSD internally with a Mac is risky since no one supports that configuration (Apple won't help, the M.2 SSD manufacturer won't help, and the adapter manufacturer....who knows). I do know that OWC does make a drop in replacement and OWC provides support for that SSD when installed internally in a Mac.

Good luck.

Least resistance would mean Big Sur if that is what Crucial is recommending for compatibility with the P3, or going with the external P3 solution with Monterey.