During disk writes, most comm programs drop the RTS signal to the modem. Also, make sure your communication program drops the RTS signal when writing to disk. This is an option that must be selected in most programs. Errors such as these during disk writes can happen in Windows sessions as well. In this case, you may need to disable smartdrive or your disk cache before downloading.
Line noise causes errors to data packets during downloads.
Poor connections are usually the cause. Most protocols can recover from this, however some take longer than others. If you have a bad connection, use xmodem as the download protocol. On a noisy circuit, xmodem is still the best and fastest way to move data. A loss of carrier can happen for many reasons.