Internal drive not mounting
I have a MacBook Pro with an internal SSD running OS 10.13. I have an external drive with OS 10.12 on it.
When I boot from the internal drive everything behaves normally.
When I boot from the external drive, the internal drive doesn't mount. Disk Utility shows that it is there but the Mount button is grayed out. I can't figure out what's going on.
I have a feeling the reason/solution is something obvious that I'm missing. Any ideas?
Thanks.
MacBook Pro