Switched from PC to Mac and terrified of screwing up my library switching to Aperture

I recently bought my first Mac so I'm already learning a new systems, finding buttons in the "wrong" places etc. But to add to that, one of the main reasons I finally made the jump is that I really wanted to give Aperture a whirl. I have been using Lightroom 3 for quite a while.

I have about 70,000 total digital photos. 60k of those are non-point-n-shoot photos that I kept in a single library in Lightroom on my PC. Photos are fairly logically organized from a directory tree structure.

I have two primary concerns... because my photo collection is so large, I keep it on an external drive (it literally won't fit on the 320GB drive that came with the Mac Mini). In Lightroom, the database and the files didn't have to be in the same place. I kept my database in My Pictures, on the local drive and the actual files were on my "Z:\" drive. Should I put the Aperture library on the Mac's internal drive or put it with the photos?

Secondly, is Apple going to move all my files around if I tell it to let Aperture manage my library?

Who has made the switch from Lightroom to Aperture?

Aperture will not be able to read any of the LR settings / catalogs / whatever.

So all changes you made in LR are gone.

You can of course save metadata to file, so at least keywords etc are stored and read by Aperture, but that is it (as far as Raw files are concerned).

I have both Aperture 3 and Lightroom 3 running on a Macbook Pro. You should be able to leave all your images on the external hard drive, and import as a reference file. The same with new images. Aperture can either import and manage the images internally, or import as a reference, and you can store them anywhere you like.