Regarding your question about previews:
That is a matter of taste: patience vs. performance.
Previews is something that LR will create on demand, e.g. when you look at one image in Library-Loupe view.
When you do so the image needs to be online, so if your three external HDs are not plugged in this will not be possible.
Personally I prefer to have all previews with me on my laptop, while the bunch of my images resides on a network storage, so unavailable when I travel.
But thanks to my previews I can still show offline images.
For this I have specified my standard size fairly generous (1680 pixels, preview quality high - you do this in catalog settings, file handling).
When doing backups of my Lightroom stuff I always exclude the previews folder, as this 95 GB big for a catalog of 92'000 images.
LR can rebuild it any time, albeit this takes many hours, more than just 1 night.
I would perhaps ditch your LR2.7-previews since I do not know if LR4 can directly make use of this version any more
and invest the hours to let LR4 build new previews (Select All Photographs from Library, then menu Library - Previews - Render Standard Sized Previews). LR will scan existing previews and build ones for the rest. You can work in parallel with LR, but it may be slowed down due to this activity.
When you have verified that your last 2.7-catalog is ok to open it is sufficient to just keep this version a while safely stowed away.
Same for your future LR4 backups: verify once in a while that they open ok and then delete the older ones.
Wow, Cornelia! Lots of good info and advice and I've saved your notes to my 'brain food' file. I was pretty sure the 2.7 previews were now obsolete and thought that the 2.7 catalogs would not be of any use with LR4, too. So far, I've never had to revert back to an older LR program - knock on wood.
Interestingly, my LR4 previews .lrdata files are more than TWICE the size of the LR2.7 .lrdata files. Curious? I can see that the previews were carried along with the catalogs when I upgraded - but TWICE the size now??
Anyway, I'm not sure I really NEED to know WHY . . . my old brain doesn't need to be overwhelmed by a lot of stuff I don't really NEED to know!!
Thanks for your help, Cornelia, much appreciated.
PS Fun? YES, THAT IT IS and I am!