An external HD ( presumably USB ) will display the contents when
plugged in and that's normal behaviour. If you are getting this
happening at random whilst the drive is plugged in then I'd say that
the USB connection is loose and breaking contact / re-making so that
Ubuntu believes the drive has been removed then plugged back in. So
check the connections either end plus the power lead to the external
drive if it has one. If the external drive is one that is self powered
there may not be enough power being supplied and often these have a
USB cable with a dual-plug to provide this extra power.

As for the 'tower of icons' this is the Unity desktop and is how it's
designed to be so cannot be removed. You could install the KDE desktop
and use that instead as this has a look that's closer to what you've
been used to in 10.04, but it's not the same.

Personally I prefer KDE over Unity and have switched to Kubuntu which
is the KDE version of Ubuntu so there's no 'Unity' interface at all.

Unity is fine for the light user but too restrictive IMHO for anyone
wanting to to more actual work.