LR 2.5 64 bit will not open catalogs from LR 2.5 32 bit

Question

I just upgraded my PC from XP (32 bit) to Windows 7 (64 bit) and have changed LR (version 2.5) from 32 bit version to 64 bit version in the process
I have used LR 2.x for a long time on my old system, and had created a good archive structure with LR catalogs for each year on an external drive + had one for the current year on my computer.
After the upgrade, LR will open the one catalog that was stored on my computer (but now transferred to the same disc as the archive structure), but not the catalogs in the archive structure. The error message says that the archive catalogs are not writable - even if these catalogs are exactly the same and have been made in the same LR version as the one catalog that will open.
What's wrong here, and - most impotantly - what's the trick to fix it?
Here is the error message that I am getting on the archive catalogs:

Answer 1

While you are NOT running Lightroom, look in the directory where your catalogue is located and check to see if a file with a ".lock" extension is there. If so, delete it and then try running Lightroom again. That file is created when Lightroom starts and is deleted when LR exits. It prevents two copies of LR from accessing the catalogue simultaneously. If LR crashes, that file can be left on disk, preventing access.
Hal