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You have pointed out what I consider one of the primary shortcomings of the Sync Services framework: the lack of a 'fail-safe' mechanism to prevent the propagation of what amount to 'null records' when a disaster happens. You know - the disaster you do not, or cannot, see coming because it occurred elsewhere, unbeknownst to you.
There are several approaches, but the simplest one would appear to be following a set of procedural 'rules' something like this:
• designate a
primary device [in your case: the Power Macintosh G5]
• set the primary device to Synchronize with .Mac: Manually
• synchronize all other Macintosh computer with .Mac at a regular interval
• use the Missing Sync and your Treo only with the primary device
• back up data records on the primary device at regular intervals using a multi-generational scheme, to different storage locations.
That limits the likelihood that data error conditions elsewhere will damage the records on the primary device. The obvious drawback: changes made anywhere other than on your Treo or your Power Macintosh G5 will not propagate until you trigger manual interation with .Mac, but if that follows a contemporary backup, you'll be able to recover from any unpleasant surprises by restoring records and performing a reset.
It would be nice if the truth database detected a potential disaster—say, your entire record set about to be overwritten because of a mass record deletion—and asked if you wanted to proceed with such a substantial change, but…