I have a 2012 classic MacBook Pro that crashed the other day when waking from sleep. On reboot, I got the dreaded missing boot volume indicator. Rebooting with the option key showed no available boot partitions.
I reset NVRAM (and just for kicks the SMC). This had no effect.
Booting into Internet Recovery showed no obvious problems in Disk Utility: the drive (an aftermarket SSD installed years ago) was there and tested just fine. First aid on the disk did not fix the boot issue. When trying to select a startup disk, the internal drive was not on the list. From terminal, I could navigate the drive just fine and copy files, no problem.
I reinstalled MacOS. During the setup process, the machine rebooted and when it did, I got the same missing boot volume indicator. Rebooting holding the Option key, however, showed the boot drive. I selected Macintosh HD and Setup continued. The machine was fine. However, overnight it did not go to sleep properly after copying some additional files and the battery drained to 0. On waking up, I again got the missing boot volume icon. Rebooting with option held down shows the internal SSD and the machine boots fine.
So, how can I get my MacBook Pro to correctly and automatically boot off of the internal hard drive every time without having to hold down the option key? The computer is shared, and not everybody who uses it is going to remember to do that.