Thank you for the quick response, Rohit.
I was able to Open the catalog. Now my issue is that "My Files Cannot be Found".
My presets, and all photos are not present. What's the next step?
In my opinion, the desktop is a poor choice of location for a catalog file. Nevertheless ...
File cannot be found indicates that the photos are not in the expected location. You need to find the photos using your operating system's search feature, then make a note of the folder lcoation where you found the photos, and then follow these instructions: Adobe Lightroom - Find moved or missing files and folders
I'm a lost here. My recent catalog is saved on my external hardrive but when I click on it I can't open Lightroom. It will open from the desktop. Before I follow the instructions you sent me. Do you know where I can get instructions on how to find my photos via the operating system? This is all Greek to me. Do I have to go through every single collection I have on my catalog to find the images.
And presets.. Do I need to plug those in again?
The main problem is going to be that LR remembers the path to each photo. On a PC this path probably starts with a drive-letter. On a Mac the path likely starts with a device name for your external drive. Since these two designations for the external dive are different you will always have missing photos when you switch from one OS to the other.
If you are transitioning from the PC to the Mac and will abandon the PC then converting all the paths from the PC way to the Mac way should be easy enough but it's not going to be convenient to do this every time you switch from one computer to the other although it can be done.
The general instructions about how to Find Missing Folder are in step 6, here:
BTW, I think the problem opening your catalog from the external drive may be that your Mac only has readonly access due to the type of format of the drive. You can copy the catalog to your Mac, but then if you make changes you won't be able to copy it back to your PC if the hard-drive is readonly from the Mac's point-of-view. You also may not be able to work with the photos if they are readonly.
I'm a PC person not a Mac person so I don't have enough experience to understand if there is a simple way to host your files on an external drive and have it work on both a PC and a Mac interchangeably.
What I am doing now is opening LR then trying to open my catalog that I have saved on my WD My Passport external drive. The error message I receive is: LR cannot launch with this catalog. It is either a network volume or on a volume on which LR cannot save changes.
Do I need to reformat the external hard drive?If I do can I use it on my PC?
Are you moving LR to the Mac or are you going to be using LR on both the Mac and the PC? If the latter, how often will you be swapping, every few minutes, every few weeks, how often?
There is commercial software that can allow your Mac to write to an NTFS drive which the external drive likely is:
If you format your external hard drive then you'll lose all the data on it. And if you format it in a Mac then you probably can't read it on a PC. Leave it NTFS like it is, now, and use the product(s) described in the article linked above.