Using external drive for Lightroom catalog.

I think you want the PHOTOS to be on the external drive, and you can then leave the catalog file on the internal drive. It is the photos that take up the largest amount of space. If you want to move the photos to the external HD, here are instructions (scroll down to the section entitled "Part 2 -- Updating Folder Location". Adobe Lightroom - Find moved or missing files and folders

If this is a NEW SSD External drive it is more than likely Formatted in the Windows NTFS format. 99% of drives sold are natively formatted for Windows (Unless specifically stating it is formatted for Mac OS X).

Apple Mac OS X can READ NTFS drives but can NOT Write to them.

So the first thing you need to do is Format that SSD in the OS X format. You do that with Disk Utilities which is found in the Utilities folder in the Applications folder on your hard drive.

Once that is done there is no need to create a New Catalog. Just copy over your original catalog and all your images from your internal drive to the external. Then Open that catalog and Relink you images from where they are now stored on the external.

Once you have that done you can move to Trash the catalog and image files on the internal drive.

Sometimes an external SSD drive is seen as a removable drive and Lightroom cannot store a catalog on a removable drive. This seems to be the problem here. It might be permissions, like Jim says, but the error suggests it's the removable thing. I'm not sure how to tackle this problem, but it must be something at the system level. Maybe reformatting it with Disk Utility can solve it. Also check how it is formatted currently. MacOS X can read from an NTFS formatted disk, but it cannot write to it.

I would think there's a permissions problem with the external SSD.....all my catalogs are on an external SSD with no problem, so check the permissions of the drive.