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I have a 2015 MacBook pro that I am in the process of refurbishing. I purchased it used and it was missing many parts (wifi card, IO board, SSD). I have replaced all of these parts and purchased an aftermarket SSD (it looks like an M.2 with an adapter on it).

The machines internet recovery wants to install Yosemite so I assume this machine is on an older firmware version which predates support for NVME drives.

As far as I understand I need a boot rom upgrade that comes with high sierra in order to boot/use the aftermarket disk. I tried installing Mac OS on an external disk and upgrading it to High Sierra, but I ran into a firmware validation error.

Since apples boot rom/EFI firmware upgrades are staged on the internal disk, it seems this upgrade path is impossible without an original proprietary apple drive.

Is there a way to manually install the boot rom/EFI firmware upgrade in this situation? or is my only option to purchase (a very expensive) original apple drive?

Update: I have personally resolved this by sourcing an original drive, running the upgrade, then swapping in the new drive. I will accept an answer to this question if a feasible alternative is proposed.

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Why not try the (alternate) command and (better) restore using latest macOS?

https://support.apple.com/en-us/HT204904

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You can't install via the (alternate) and (better) restore method if your Mac doesn't have APFS-supported firmware. It's right there in the link notes. Then your recovery gives you the original you shipped with (in my case, Mavericks), and there is no way between here and the moon you can get a boot rom update to work if you don't have an internal SSD. I've wasted a ton of time on this. I would think that someone had come up with a solution as installing 3rd party SSDs is fairly common and there must be some large number of us who are doing so because the original is completely dead. So, I also had to source a used original Apple SSD. It's beyond annoying, but $40 versus 5 hours of messing with this seems like a good trade.

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