How can I use the same catalog on PC and MAC?

Question

I keep my catalog and photos on an external hard drive.  I have both PCs and a MAC and I would like to be able to access the same catalog of photos on both my PC and my MAC using LR2.  I am using v 2.3.  The problem is the location of the photo is referenced differently on the PC than on the MAC.  Does any one have any idea how I might accomplish this?
If this is impossible, then can anyone at least tell me how I can convert my catalog file (lrcat) so that the location of photos that are currently referenced using a drive and path (eg, p:\myphotos\...) to a format that can be used on the MAC so Lightroom running on the MAC will know about the photos on my external drive?

Answer 1

I have figured out the answer to my question.  Thanks to those who tried to help.
First of all, I found that Lightroom's 'Locate folder' didn't work for me.  It might have if I had tried that the first time I tried to convert things, but I had tried to locate a few photos already, so I was always told that the folder I was trying to locate was already in the catalog.
Breifly, what did work was to locate individual photos using the ? you see on the photo when in grid mode in the Library view and specify that other photos nearby should also be relocated.  By choosing a photo at the top of the directory tree, Lightroom will search for all photos in the directory containing your photo and all subdirectories of it.
The details:
What did work, was to individually locate photos using the ? you see on the photo when in grid mode in the Library view.  If you turn on the choice to locate other photos nearby, then Lightroom will search for other photos in this directory and in all subdirectories of the one containing your located photo.  This was a good start, but the problem for me is that my photos are stored in many directories below an organizing directory (eg, \2009\2009-01-02) and the organizing directory (\2009 in this example) does not contain any photos to locate.  Furthermore, each of my orgainizing 'yearly' directories are organized in one top directory (\Photos) that also has no photos in it to locate.  So, to make use of this feature, I had to create a photo in at least the top directory that I could use for finding the rest.  I call this a 'FindMe' photo.  I found that it needed to have a totally different file name than any other photo in my library.  So, I created on be exporting another photo (with very poor resolution to keep the size small) and giving it a name of FindMe-<directory_name>.  I also found it helpful to give it a caption of 'FindMe' to use in locating the photo.  I imported this photo into the top directory (\Photos) on the PC.
The next step was to open the .lrcat on the Mac.  Looking at the grid of all photos, I narrowed down the photos to just those that had a caption of 'FindMe' or a file name of FindMe.  That located the photo in \Photos that I could use to start the search for all the rest.  (Note that giving your locating photo a name you can easily find is important because all you will be able to see when you look at all of the photos is just a blank square and I couldn't find any way to tell Lightroom to only show me photos in this one directory, excluding subdirectories (this is a feature they should add to Lightroom).)
Now theoretically, this trick should have located all of my photos.  However, in a library of over 55,000 photos, there's bound to be some problems.  It turns out that my library that started in Adobe Photoshop Elements had some entries that didn't point to the file properly...in fact there was a blank space at the beginning of the filename.  We had caught this problem and renamed the file on the hard drive properly, but a few catalog entries had missed the correction of the filename before we converted this to a Lightroom catalog.  (Elements tolerated the blank space in the file name, Lightroom does not).  This caused Lightroom to hang when it was locating file.  I solved this problem by fixing a lot of these in the PC catalog.  (This took some doing, because Lightroom would first give me a ? to let me locate the file, but it quickly realized it had a problem and changed the ? to a danger sign that would not allow me to do anything with the photo.  So, I had to act quickly to correct the problem with these photos.)  That fixed a lot, but there were apparently still some problems in the library that caused Lightroom to hang when locating photos.  Also, as the number of photos that were properly relocated grew, I found that it took too long to let Lightroom try to fix the entire library.  So, I started creating FindMe photos in the few remaining 'yearly' organizing directories and locating files in each of these.  That pretty much did the trick.
My wishlist for Lightroom new features:
1.   I would like it if I could tell Lightroom to locate all photos within a directory (with the choice to includ subdirectories) in a particular place.  By this, I mean to go from the directory, not from a specific photo within a directory.
2.  I want to be able to tell Lightroom to only display photos within a particular directory (excluding those in the subdirectory).  This would make it a lot easier to locate my 'find me' photos and would eliminate the need to make the so identifiable.