MacBook Pro will not mount/boot to external SSD

So far, I’ve only been able to upgrade to El Capitan (I don’t have access to the Sierra installer, only High Sierra). Every time I try to upgrade to High Sierra, there’s a Firmware error, which I was assuming was because the SSD was formatted to HFS_+ instead of APFS.

Well, I reformatted to APFS and it didn’t work. Both using GUID partition…

Side note: I do Not have an internal ssd in this computer. This may be the issue entirely, but figured there has to be a workaround somehow?? Don’t really feel like shelling out the cash for their almost proprietary M.2 SSD..

I’ve tried most of the routine stuff:

• Reset NVRAM
• Reset SMC
• Repair disk
• First-aid
• Unmount & re-mount Disk
• Reformat ssd to APFS
• Reformat ssd to HFS+
• … None of those have worked.

Borrowed a newer Macbook Pro (2017), which has an internal ssd (with High Sierra installed), and was able to completely upgrade the external ssd to OS X Mojave AND migrate my old time machine backup to the ssd. This worked like a charm!

Well check this out - plugged the ssd into my late 2013 Macbook pro - it didn’t even see the drive as bootable... This is regardless of whether the ssd is formatted to APFS or HFS+ using GUID partition…

On to the firmware:

The EFI partition shows up in terminal when using the “diskutil list” command.

I’m to the point of wanting to go ahead and apply a firmware update via terminal. I’ve done plenty of firmware updates on Windows based computers, but never a Mac product, so I’m not sure if they’re as stable (as a Windows firmware update) when done manually.

Hopefully we can fix this issue!

Thanks!