Can't add files to external drive after Yosemite update - HELP!

I'm using a Mid 2011 iMac and updated to OS X El Capitan (Version 10.11.6) last week. Before that I was using an external hard drive to work with Lightroom to save space on my computer. Since installing the update I can no longer add anything to this external drive (or any other for that matter) so I can't add new photos. Can anybody explain to me how to get the drive back to what it was originally like without losing all the data on it please?