having difficulty moving my photos file from one external disk to another

Check the file system format of your drives. Your new drive should have the file system format "MacOS Extended (Journaled)", and it must not be case-sensitive. And the drive must not be used for Time Machine backups.

I take it, that your old drive had a case-sensitive file system format and your drive has not a case-sensitive file system format?

. "You can’t copy “Photos Library” because it has the same name as another item on the destination volume, and that volume doesn’t distinguish between upper- and lowercase letters in filenames."

Have you tried to rename the Photos Library you want to copy to a filename, that is different from any existing name on the destination drive? The letters need to be different and not only differ in upper-case and lower-case.

After renaming the library try again to copy it. If you have cloning software (Carbon Copy Cloner or Super duper for example), then you may want to clone the library instead of copying it.

Added: If you do not have cloning software, and if you still get error messages when you try to copy the renamed library, copy each subfolder of your Photos Library individually - try to copy the the library in smaller blocks. When yo copy the library in parts, you are able to identify the culprit that is causing the I/O error and can skip it, when copying: Be very careful not to modify the original library when you try this.

To open the Photos Library package and to copy the sub folder individually do this:

• Create a new folder on your external drive. Call it "Photos Library Copy".
• Ctrl-click (or right-click) your current Photos Library and use the command "Show Package Contents". This will open a folder, showing you the files and folders inside your Photos Library.
• Copy these folders one-by-one to to the new folder on your external drive. If one of these folders is giving an error -36 or similar, check how far the copying proceeded. The files, that have been copied, are o.k. The file or folder, that should have been copied next will be the unreadable file. Probably the problem is caused by one of the files in "Masters" or a database folder. In that case look into the Masters folder and copy as much as you can, while leaving out the file that will not copy.

When you are done, rename the copy of your Photos Library to "Photos Library Copy.photoslibrary" and double click it to open it in Photos. Can Photos open this library?