First backup all the files and your Catalog!
Say you have a 2010 folder and contents on drive X, and another 2010 folder and contents on drive Y, and the internal arrangements within each 2010 are essentially the same (consistent). And now you want to proceed with everything merged on drive Y.
The method I'd use, after backing up, would be:
- Use Explorer / Finder (outside LR) to move folder 2010 bodily from X, into the same parent folder where 2010 is located on Y
- The operating system will detect that some folders (including 2010) already exist, and ask your confirmation for merging folder contents - that's what you want.
- [Any conflicting individual files will be notified and you have the opportunity to skip those from being moved. Skip so that LR's addressing is not confused. Hence they will stay behind in drive X, along with the needed folders to contain them, after everything else has transferred. You'll need to review and make separate provision for these, but for now move these temporarily to another location entirely.]
- Open LR. In the Folders panel you see the images in drive X and the images in drive Y. The images and folders in drive X now show (?) offline because of the external move you have just done.
- Go to the "2010" within drive X: and right-click (cmd+click) and select Find Missing Folder. Navigate to the 2010 which is on drive Y.
- Lightroom re-connects latest edits etc, onto all the images you have just moved across.
- [If there were clashes of filenames found: move those back where they were on drive X. Lightroom still refers to those. Now you can rename things as needed (within LR) to avoid the clash. Then repeat the above process.]