Crucial X9 disk will not mount if MacBook is not powered.

Hi,

Yes, you need to powered on your MacBook Pro to mount the external drives.

It seems to me, you need to get NAS, Network Attached Storage. You can access your pictures data on the drive from your any Mac or PC you have via network 24 hours a day, 365days a year with or without your MacBook Pro is powered or not.

The bus-powered Crucial X9 will mount if macOS is running and the computer is powered by its battery or its A/C adapter. The X9 is preformatted as exFAT and should mount, even if you have not placed any files on it. Failure to mount would be using a wrong USC-C cable (it comes with a USB-C 3.2 Gen 2 cable), or the drive has somehow become corrupted or damaged. It would have to mount in order for you to use Disk Utility to perform Disk First Aid on it.

Have you used a non-Apple drive utility or previously formatted it on a Linux system where that Linux filesystem would not be recognized by macOS?

I have used it to store several iMovies already. I think when grandchild arrived I unplugged it without ejecting it. Now I can only see my slower x6 fine when I plug that in. But no x9. Most annoying since I've now cleared the iMovie library from the Mac!

<< It would have to mount in order for you to use Disk Utility to perform Disk First Aid on it. >>

The requirements to allow Disk Utility Repair is very slightly different than you stated.

If the weird case that drive has a Volume, but its directory is damaged so that the drive will not Mount, you MUST perform a Disk Utility Repair to Fix the Directory. You can indeed perform such a repair inside Disk Utility with the Volume recognized, but the Volume Not mounted. If successfully repaired, THEN the Volume can be Mounted.

In the case of a drive that is uninitialized, there is no Volume at all, and that can not be repaired unless/until a recognizable Volume of a known type is created.