Yes - I have this too.
Time machine worked faultlessly with Yosemite.
Now with El Capitan:
- sometimes the external disk fails to mount, so it is not shown on the desktop
- sometimes it does mount, and is shown, but back-ups to Time Machine fail to occur at hourly intervals, and I must initiate manually
- sometimes it works fine
-almost always, when the Mac puts itself to sleep, on awakening I get the error message "disk not ejected properly". This also happens on shut-down, unless I manually dismount the disk by dragging it to "Trash" before shutting the computer down.
I am using a G-drive external disk, which, as I say, worked fine with Yosemite.