Soapy,

Be careful with this one...
Basically you can change the ''BASE_REPOSITORY_PATH'' parameter in the Site Configuration parameters.
This requires a cycle of the QC/ALM service to get QC to use it.
the next time you look at the Physical Directory property, it will be updated with the new ''base'' path, then your domain name/path after it.

Here is the tricky part: If you just change the base path, the ALM service won't start, it needs most of the other files/folders that are immediately below the base repository path in order to load up correctly. You are best to just copy the whole repository folder contents (including all domains and projects) to the new location to get it all -- this would likely take a long time.

You cannot change the Physical Directory of a Domain independently of the other domains -- they all get the SAME base path for the repository. (By the way, QC works best with full UNC names if using a network location - not a mapped drive).
So, if you REALLY want to have a domain isolated from the rest of your repository -- you cannot do that with ''BASE_REPOSITORY_PATH''. You can, however, locate a specific project's repository folder anywhere that is accessible from the server (use remove, edit the path in the DBID.xml file, then restore). It would be confusing to have too many projects scattered around on the server like this, but this is helpful when you want to migrate a few projects to a NAS location, for instance.