Could you please check if you have Read and Write permissions on the external drive.
Once you add the Read and write permissions to your name , please try and open the Catalog in Lightroom.
Hello! I just spent many hours figuring this one out. If you have a Seagate external hard drive and it is formatted for Windows NT File system, then you have to get the updated Paragon Driver for Mac OS on the Seagate website. Download and install the driver (it's free) and then the write access will be restored. I had only read access after updating to El Capitan and now I have both again.
To see how the external drive is formatted, go to Finder, then click on the drive and in the gear dropdown box click on Get Info. That will tell you how it is formatted.
So far, this has only happened with the Seagate drives. My SanDisk drive was formatted for MAC only so I didn't have a problem with that one.My Seagate drives are formatted for Windows NT File system so I can use them on both my Macbook Air and my Windows desktop PC.
I hope this solves your problem. Good luck!