http://literature.rockwellautomation.com/idc/groups/literature/documents/pm/1756-pm015_-en-p.pdf
"The controller keeps up to 100 log entries buffered in its internal memory. You
can configure the controller to write the buffered entries to the removable media
when its internal buffer is 80% full. Additionally, you can command the controller
to write the buffered entries to the removable media with a message instruction.
This procedure is detailed below. Once a log entry is written to the removable
media, it is removed from the buffer.
If the removable media is not present, is full, or if the controller is not configured
to automatically write buffered entries to the removable media, and the internal
buffer becomes full, entries continue to be saved in the buffer in a circular fashion.
As new entries are stored, the oldest entries are discarded."
It should not loose your project by filling the buffer.
Regards,