MacBook Pro 9,2 can't read an SSD with a macOS High Sierra OS Good day everyone.

I have a Macbook Pro 9,2 that can't read an SSD that has a macOS High Sierra installed on it.

For the background context, a friend gave me her Macbook Pro for me to retrieve the files, and told me I can keep the laptop after. What I did was I took out the current hard disk, inserted it on an enclosure, and gave it to her.

Now, I have two "spare" SanDisk SSDs:


240 GB -- macOS Sierra
480 GB -- macOS High Sierra


At first, I inserted the 480GB SSD, but it wasn't booting. I was given the blinking folder icon which indicates that the laptop wasn't able to read the hard drive. I then rebooted the SMC and PRAM, and on the normal boot, I held the right option/alt key to see the bootable devices, but I didn't see any, which established that the laptop couldn't read the 480GB SSD as a bootable device.

I then inserted the 240GB SSD, and I was able to boot normally. I then inserted the 480GB into an enclosure, and then attached it to the laptop. The laptop was able to read the contents in the Finder, as well as list down the item in Disk Utility. However, the top level was read as AppleAPFSMedia, and then the SSDName under it. This gives the impression that the laptop cannot properly read the 480GB SSD as a bootable Hard Drive.

Now, we how will the MacBook Pro read the 480GB SSD APFS as a bootable hard drive?

Right now, we're thinking of upgrading the 240GB SSD from macOS Sierra to High Sierra. We think that doing so would upgrade the BIOS, and enable the laptop to read APFS format hard drives. However, we're very reluctant to do this, because if it doesn't work, we will need to wipe a hard drive and try to install High Sierra again.

Has anyone experienced this situation before? Any lead would be very much appreciated.
Okay, if you have a macbook pro that works on an older OS (pre High Sierra), and you want it to boot from an SSD (SSD1) that has a High Sierra, you have to make it read an SSD (SSD2) that has Sierra at the most, and then upgrade that to High Sierra.

Once the upgrade is done, you can slap on SSD1 and your laptop should now boot on that drive.

I'm not all too sure what happens here. We just jumped the gun and updated to High Sierra and it fixed the problem. We aren't so sure if installing the firmware updates would have helped.