In LR: unless your new drive will have the same drive letter or device name as the old one, then to prepare for the copy or move, you want to have as few parent folders of the existing photos as possible so use Show Parent Folder in LR Library on the old drive so all the photos are under one or a few folders.
Outside of LR: copy all the folders from the old drive to the new drive using the OS so that the same relative folder structure is used. You would want to use the OS functions like Explorer or Finder or some sort of copy utility, to do this. Don’t do it in LR, because as you say you might miss something. I would also recommend copying not moving in case something goes wrong you still have the old copy until you decide to remove it.
Outside of LR: after the copy, either disconnect the old drive, change its drive letter, or rename the top-level folder(s) so you can’t access the folders accidentally in LR.
In LR: each top-level folder will have a question-mark and hover-text that says it’s missing. Right-click on each top-level folder, select Find Missing Folder and browse over to the new location of that folder on the new drive and click Select Folder. Repeat for each top-level folder.
In LR: after you’ve relinked everything, if you did Show Parent Folder before, you can reverse it by using Hide This Parent (folder) so that the lower-level folders are at the top level.
That should be it, unless you’re also moving the catalog to the new drive, then you’ll want to move presets and other things that may or may not be in their standard locations.
Thank you for your answers. Let me explain what I'm trying to accomplish and ask you a couple follow up questions, if I may.
I'm planning to move the entire library off my internal HD to a portable drive (a WD My Passport) so I can access the library from two computers. One is my current Windows 7 desktop PC, the other is my wife's MacBook Air. The new portable drive, formatted as EXFAT, will connect through the USB 3 ports on either machine. I've read that this is the recommended approach when one wants to access their photos from more than one machine.
Are there any particular data integrity issues I should be aware of? I'm already aware that the catalog and the thumbnail cache should be local to that drive. Do the Presets remain with the computer, or do they go to that portable drive as well?
Thank you again,
Are you planning on having separate catalogs reference separate subsets of the photos, so you and your wife are not working on the same photo in two different catalogs, or are you expecting to be working on the same catalog of photos? I ask because if you’re both going to work on the same set of photos then having two catalogs isn’t going to work, as well as the fact that you can’t have the same path to the photos from both a PC and a Mac and not have to do a Locate Missing Photos every time you swapped between the two computers.
Finally, if you’re going to use an external drive then you probably should get two of them and replicate the photos from the working drive to the backup drive as often as you make changes so if you drop or lose or otherwise lose access to the contents of the portable drive then you have a backup copy safe and ready to go.
There is only one catalog for the entire library and that catalog will be local to the portable drive. Also I have a backup system in place so there will always be a copy if the drive is lost.
I've copied my Photo Library (the folder that contains all my original RAW files) using drag and drop in Windows Explorer. When I select Properties, the file counts match. The exact size is ever so slightly different (about 3,000 bytes out of 500Gb).
Is it safe to assume I've got it all?