First of all check, that there is not a message displayed on the mobile device mentioning a particular problem.
If you synchronize mails and chose too high values for the maximum size of the mail body and attachments this could lead to a synchronization error and a disconnection of the mobile device. Even if you use an external storage card, the memory of the Pocket PC limit the size of the attachments at the present time. It is suggested to try values inferior to 500K.
If the mobile device disconnects and that you can no longer access to its files through the explorer, check that there is not a problem with your USB cable (bad connection). If it is connected to a hub, try to connect your mobile device directly.
If the problem persists, send the logs to support [at] birdiesync.com with "Tools/BirdieSync/Archive logs..."