LR knows the "absolute" path to each individual file. So, the files must reside at exactly the same places, by name and folder structure, on the new computer. Your missing images should have a small exclamation point in the upper right corner. If you click that icon, you should get a pop-up to 'locate' the file. In that window should be the old location that tells you where to go to find the old file on the old computer. If these images have already been copied to the new computer, you can use the <locate> button to navigate to that new location. If you work from the All Photographs view of the Catalog, when you update the location of one image, all images in the same folder will be updated automatically.

The "best" external MECHANICAL drive can be either a Toshiba or HGST Ultrastar - purchased as a bare drive. These are very robust drives rated for data center or enterprise use - and come with 5 yr warranties. You purchase a case that offers the fastest connection supported by your computer (Thunderbolt2, USB 3.0/3.1/C) and take the 90 secs it takes to install the drive in the case you will end up with something substantially better than the cheap pre-packaged, so called portable or external, consumer grade drives. If you open one of them you will find the mfgr's cheapest slowest drive inside. These represent a total waste of money.

If you are able to justify a much higher cost - an SSD drive is also a very good option, but don't think that because it is an SSD it will be much faster than the mechanical drive - it won't be because the speed is limited by the interface bottleneck.

The "best" external MECHANICAL drive can be either a Toshiba or HGST Ultrastar - purchased as a bare drive. These are very robust drives rated for data center or enterprise use - and come with 5 yr warranties. You purchase a case that offers the fastest connection supported by your computer (Thunderbolt2, USB 3.0/3.1/C) and take the 90 secs it takes to install the drive in the case you will end up with something substantially better than the cheap pre-packaged, so called portable or external, consumer grade drives. If you open one of them you will find the mfgr's cheapest slowest drive inside. These represent a total waste of money.

If you are able to justify a much higher cost - an SSD drive is also a very good option, but don't think that because it is an SSD it will be much faster than the mechanical drive - it won't be because the speed is limited by the interface bottleneck.

I wanted to upgrade from my Canon point and shoot so after performing due diligence, I decided to buy a Nikon 5600 despite never having owned a replaceable-lens camera, my ultimate goal being macro photography. The camera having duly arrived, I am certain I am in over my head. The vast amount of potential activities and all the bells and whistles and the PDF manual delineating how to perform them have served to make me wonder if I should return the 5600 and opt for a Nikon 3500SLR which I have been assured is a far more simple camera for my intended purposes. Would appreciate thoughtful opinions on this subject.

I have recently gone through the same process as you are now considering. Apple user. MacBook Air and needed to upgrade for faster processing given the size of the images (D810 then to Z6). 1st-- go with a new pro, since the processor speed tests far exceed the older models and Airs. 2nd-- I would keep my pictures on external drives and just keep your catalogs/libraries on your main drive (everything backed up of course). If you do this then the 512GB should be fine. 3rd-- We are almost to July and Apple is going to announce a new MacBook Pro in the fall. I personally would wait for 2 reasons: maybe the new laptop will be so outstanding that I would want it; or if not, the prices for the 2018s will drop. Check out the chatter about the new Pro. In any event, only go with a minimum of 16GB RAM with whatever you do. Something else to consider....check out your software (all your normal everyday software) to see the compatibility with Mojave. While most software will work, some are not 'optimized' for use. And some will not work if they are older 32bit programs. It is a good probability that the new Pro will ship with Catalina. I don't know how this will affect our older programs--not in a good way though.

Super helpful, JBK. I’ll have to check the release date on the new MacBook Pro. I thought it was already available. If not, I’ll limp through to fall. Thanks! JDM
Last, there is quite a bit discussion about the Touch Bar. I used it very little, but am finding it useful as I work more and more. It is/was not an issue at purchase. I wanted the latest processors; 15" screen, and best display offered.
Good luck.