OK, now I'm confused.
So in Finder what's the difference between dragging the "Aperture" lens icon into the external HD or the "Aperture Library" icon (stack of photos with a pink flower)?
Well I dragged the pink flower icon over earlier and got prompted to upgrade something, got into a muddle and found this old version of Aperture, then thought I'd lost all my photos. There was a box with a "tip" to hold down the option key as I opened Aperture, which I did, and got the box below, which I've never seen before.

The one highlighted is up to date with 1885 originals. Highlighting the one above that (saying current default), shows only 588 originals and it's on the Mac HD>Applications (see below)

When I highlight this and click "choose", it's just a load of old stuff, basically as my Aperture library looked about 2 years ago when I had the previous version of Aperture including things I thought I'd deleted.
Obviously I know what the Mac HD is, but what is the little house icon with my name after it in the first screen shot holding the 1885 originals?
Do I have 2 libraries in different places?
I'm sure this is basic stuff I should know, so sorry if these are daft questions!
Jon