You can't share libraries, but you can keep all of your albums, edits, and other metadata. Here's how:
1. Create a new Lightroom Classic catalog when you first launch the application. By default the catalog will be created in the Pictures folder. This is fine if you have plenty of space on your startup drive.
Don't import any photos.
2. Open the Lightroom preferences, Lightroom Sync tab. Choose a location for your images. The Pictures folder is selected by default. You don't want this because your 50K images will fill up the computer's startup drive.
3. Change the location to an external hard drive. I recommend buying a new hard drive with lots of storage space. Here's more info about syncing: Scroll down to "Set Lightroom Preferences". https://helpx.adobe.com/lightroom-classic/help/lightroom-mobile-desktop-features.html
4. Turn on Lightroom Sync, found in the cloud icon (upper right corner).
5. Leave it run for awhile. All of your images in Lightroom's cloud will download to dated folders on the external hard drive. All metadata is added to the catalog. Lightroom's Albums become Lightroom Classic Collections.
After the sync is complete, you can delete the images in the cloud.
6. Import new images to Lightroom Classic, not Lightroom. Make sure new imported photos are copied to the external hard drive, not the Pictures folder.
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