Photos OSX not importing videos

In addition to plugging the drive into the other machine you have to set the "Ignore ownership on this volume" for the external volume, as described here: Move your Photos library to save space on your Mac - Apple Support

Or you will have to repair the library every time after using it on another machine. The only convenient way to share a library between two machines offered by Photos is iCloud Photos - each machine is using a separate library, kept in sync by iCloud.

I have my photos directory stored on a network drive

The Photos library must be stored on a drive formatted Mac OS Extended (Journaled) or APFS. If you use an external that drive should be directly connected to the Mac (read: USB or Thunderbolt). It cannot be stored on a networked location. Storing it in an inappropriate location risks dataloss, often silent that goes on for quite a while before becoming obvious.

Your first priority is to move the library to a supported location. Frankly, if using your NAS is a key factor, then you need to use another app.

Also, storing the library in an inappropriate location does give rise to issues importing and exporting, and while Photos is not designed to support all - or even most - video codecs, moving it to an appropriate location might well solve this issue for you.

No, but these are the kinds of issues that arise from using the NAS.

Try use a back up utility to copy the library over, something like SuperDuper or Carbon Copy cloner. These will take care of any permissions issues that arise.

The only way to do it Is to physically plug the drive into the other machine. Photos is not designed with that usage in mind.