Howard, that's a different point entirely. Just in case it wasn't clear, I am putting on Dropbox only the Application Support folders.
Putting the lrcat on Dropbox might work, because the file is stored locally - only when it is closed does the Dropbox app copy the file up to the cloud. I think there is some risk, perhaps of corrupting the catalogue, but more probably of opening a slightly older version of the lrcat file on one's other computer and soon not being sure which lrcat version contains the most up to date metadata. The previews lrdata folder would also go to Dropbox as well, and that's potentially much more data. I wasn't successful in using the symbolic links technique to put the lrcat on Dropbox and move the lrdata to a purely local folder. So basically I saw too many shortfalls to make it worth pursuing these options.
John