In case anyone else runs into this issue and wants to avoid a headache: I backed up all of my files and a VM to an external drive and was in the process of encrypting it when I installed El Capitan. Once El Capitan installed, the external drive would not accept my unlock password. I wasn't sure if I forgot the password or not. I was finally able to get into my drive by installing a Yosemite VM and transferring the files from the VM to my host El Capitan machine. (I don't have a second Mac, but it would likely be quicker to unlock the drive via a separate non-El Capitan mac.) I will leave it connected to the VM to allow the encryption process to complete and see if it will then appear under El Capitan.. TL;DR. External drive wouldn't mount/wouldn't accept unlock password. Current El Capitan beta has known issues with disk utility and usb devices, so I was able to circumvent the issue by connecting the drive to a Yosemite VM (or another Mac with Yosemite or earlier).