Thumbnails building on launch, impermanent and missing several years

Question

When I open it, Aperture always wants to generate thumnails (70,000+, for a library of 190,000 referenced files).
I'm working with a fresh install of 3.4, a new plist.
If I let the thumbnails build until done, it apparently makes no difference the next time (after quitting and relaunching Aperture).  The same thing happens.
Looking at the package contents, I see that although I have pictures in my referenced library going back to 2005, the Thumbnails folder shows folders beginning at 2008.  So several years of thumnails are evidently missing.  (That's probably around the time I switched from iPhoto to Aperture as my photo organizer and editor.  That might be a red herring, or is there something qualitatively different about those former-iPhoto Masters?)
So how do I make a permanent set of thumnails for those missing years, so that I don't have to either wait a long time for the program-slowing activity to complete, or manually cancel the 70,000+ temporary rebuild every time I launch Aperture?
Thanks for any advice.
Patrick

Answer 1

Leonie,
Thank you again! 
The 2008 thumbnail folder appears to include only the normal monthly activity, but inspect files numbers in each (using command-i to look at numbers of items in each folder, in each month) I do indeed find one day in 2008 that includes multiple-thousands of thumbnails.  I conclude that it must be the date I imported a huge iPhoto library. 
That explains why I appear to have a thumbnail for each picture, going back to the first photo by date (2005), in spite of the (redundant) rebuild that wants to happen automatically with each launch of Aperture.
I intend to do your recommendation:  to manually highlight all photos prior to 2008 and generate thumbnails.
However, while waiting (expecting a long delay before I received an answer--I was wrong about that--thanks again!), I highlighted those same photos, and did a "reprocess from masters".  That will take me a long time.
Then I will try the generate thumbnails.
Then I will report back!
Even on a blazing fast machine--MBP retina, with 750G flash drive, and Masters stored on 6TB raid via thunderbolt-eSata--these things take time!  And I'm mindful to try (to change) *one thing at a time* to see if it fixed the problem, so that I can benefit from (and share with others) a simple cause-and-effect confidence that I fixed a problem.
FYI:  I have been stalled in my editing now for almost 48 hours.  Two days ago, my formerly reliable Aperture editing procedure began to freeze (spinning beach ball) while I tried so fairly routine edits on pictures.  I'm not sure the editing problem still exists, because I have allowed myself to become side-tracked by this rebuild issue, thinking I might solve my editing problem in the process.  However, the two might be completely unrelated.
I'm still trying to figure out what I changed (if anything) that might have caused the spinning beach ball.  I can think of only one thing, and it has to do with previews--but I had better put that aside for now, until I can move ahead with this thumbnail issue (or just ignore it, if I can't fix it, as I was doing before the problem occurred, without apparently affecting the editing efficiency, prior to 48 hours ago).
I have customers waiting patiently, but Aperture is stalling my ability to generate income.
Were it not for this community--and particularly your help over the last 24 hours--I would be much more anxious!
Cheers,
Patrick