MP 1,1-5,1 MP 5,1 boots up with 10.13 but not 10.8

[cinclodes]

I just obtained a Mac Pro 5,1 that came with OS 10.13. I swapped in an HD with OS 10.8, but it wouldn't boot up. I then connected an external drive with OS 10.8, and it did boot up. Finally, I tried a different internal HD with OS 10.8, and it wouldn't boot up. I cannot make sense of this strange combination of behaviors. I would like to run this computer with OS 10.8.

 

[h9826790]

Remove all other hard drive (both internal and external), then only install the HDD that has 10.8 into bay 1. Then perform a 4x NVRAM reset.

This should make your cMP automatically boot to the 10.8.

Or, if you can boot via external drive. You may boot to that 10.8, then run 10.8 installer, and select one of the internal HDD as the target drive. Assuming the hardware is good, then a clean 10.8 installation should always work.

 

[cinclodes]

Remove all other hard drive (both internal and external), then only install the HDD that has 10.8 into bay 1. Then perform a 4x NVRAM reset.

This should make your cMP automatically boot to the 10.8.

Or, if you can boot via external drive. You may boot to that 10.8, then run 10.8 installer, and select one of the internal HDD as the target drive. Assuming the hardware is good, then a clean 10.8 installation should always work.

I did all of that, and it now works. Thanks a lot!