Lightroom :: 3.6 - Previews In Preview Cache Specific To Image File Location?

Recently I reorganized my system so that my image files were on a different drive that I had previously been using a backup drive, so no additional file copying was involved. When I ran Lr it did not find any images, as expected, at the original location and I told it where to find them by pointing it to the new parent image folder. That worked fine but then it seemed as if all of the previews had to be rebuilt on the fly by Lr as I went to each folder. Could it be that the previews in the preview cache are location specific to drive/volume level rather than just folder level ?  Or is there a way to aim that ole preview cache at the new image locations ?
 
Likewise, would this apply to the ACR cache too ?  i.e. if I point Lr at the same images in the same folder names on a different drive, will the ACR cache data still be relevant or should I just scrap it and start again ?
 
I told Lr to rebuild previews for many of my images and it did so overnight, but now the preview cache is about 30GB instead of about 12GB. It's possible that I picked up previews of images that were previously not in the cache, but would the new cache still contain previews of the now non-existant images at the old locations ?
 
I have previously reverted to system backups in which the whole drive volume had previously been been copied intact. Lr always worked fine in those situations. Is my present problem because I shifted the images folder relative to the drive volume where Lr is running ?

I'm using Lr 3.6 on a 2011 MacBook Pro with Mac OSX 10.6.8.