I have a Seagate FreeAgent GoFlex drive and recently it stopped mounting on my MacBook Pro (13-inch, 2012), currently running El Capitan Beta 5. It refuses to show up in Finder or Disk Utility, but the drive itself lights up like it is connected. I have another drive that I use for Time Machine, which shows up just fine.
It works, however, on Windows PCs and OS X Yosemite latest version. I plan to submit a bug on this but I thought I'd get some community help on this issue as well, to see if there is anything else I can do to make it work.
Thanks!