Editing off of external hard drive

Whilst you CAN combine using Lightroom Classic (LrC) with the Lightroom cloud, it is better to first decide how you are going to work and then stick to that plan. There are two main methods:

1. Using Lightroom Classic as the main repository of your images, while also using the cloud to access your images from another system (such as your laptop). In this scenario you would always import you images directly into LrC, so they are stored locally on your external drive. Once in LrC you then have the option to sync selected (or all) images to the cloud, and from the cloud they would then be available to access from any other connected app/device....such as the Lightroom desktop app on your laptop, or the Lightroom Mobile app on any smartphone or tablet, or even the Lightroom Web app which is available through any browser window. The advantages of this approach are:

• You control what images are synced to the cloud.
• LrC only uploads Smart Previews of your images to the cloud, which are 2650 pixels on the long edge. Suitable for most viewing needs, and even some editing. The big benefit, however, is that they do not count against your cloud storage. So even if you only have the 20GB allowance from the Photography Plan, you can sync your entire LrC catalog to the cloud and it would not use any of that 20GB.
• You would also have access in LrC to the many functions which are not yet available in any of the Lightroom ecosystem apps.

And the downsides of this approach are:

• LrC only uploads Smart Previews, so if ever you wanted access to the full-size originals (such as for sharing) you could only do that via LrC. If you happen to be out and about with only your phone, and wanted to share an original, you couldn't do it. If you want to use the album sharing feature of Lightroom, the people you share with would not be able to download a full-resolution jpeg of the shared images, only a 2048 pixel version.
• You'd have to figure out a workflow for any image captures that you might make using your phone or tablet. Not too difficult when you have got it setup correctly, but it might need some managing if you shoot on the phone a lot and you only have the basic 20GB allowance. Plenty of users already do this, so there should be plenty of helpful hints available online.

2. The second method involves using the Cloud as the main repository for your originals. Specifically you would always only import all new images into one of the Lightroom ecosystem apps, so you would import your external drive images into the Lightroom desktop app. From there they would sync up to the cloud (full-resolution originals), where they would be handled by the Adobe servers. New captures on phone/tablet would also go to the same cloud servers. The advantages of this approach are:

• All originals stored in the cloud and managed by Adobe. No catalog or image backups required by the user (though of course there will always be some users who would also want to make their own backups).
• Access to all originals from virtually anywhere in the world, along with full edit and sharing capabilities.
• No need for a desktop computer at all.

The main disadvantage is of course the fact that some features available in LrC are not yet available in the cloud ecosystem, which puts off many of the traditional LrC user. Though of course it depends on whether you need those missing features or not. 

Now, if you want you use this second cloud-based method as your primary tool, you CAN also integrate LrC as well (e.g. to get access to those features, such as printing that the Lightroom desktop app doesn't have). You could do that by starting a new LrC catalog, enable it to sync, and a copy of ALL the images in the cloud would download into that catalog, as will any subsequent additions to the cloud. In that state there are things that you can do in LrC or Lightroom which will sync to the other, but there are some things that do not sync (such as keywords and location data), so you would have to be very disciplined in your workflow to avoid the pitfalls that await you should you adopt this particular method of working. Using this "hybrid" system is most definitely NOT recommended by Adobe, and though I use it myself I also wouldn't recommend it to anyone who hasn't got the time and experience to thoroughly test it first, in order to identify the potential issues.