Trying to figure out why the external crystal units aren't going to sleep properly for about 750ms  at a time. I don't know what the problem is yet.
Ignore previous comment about external crystal not working. It appears to be an issue with they way I have my battery power supply wired up with the oscilloscope in shunt mode, certain cards make any of the OWHLs lose timekeeping, while other SD cards make internal and external crystal OWHLs work just fine. Must be a current draw issue.
Removed the sync() and close() commands from the writeToSD function. These were unnecessary, since they forced a full flush of the data to the card every second, and each write took about 60-70ms. If you instead let the card decide when it needs to do a full flash, the average write cycle drops to 40-50ms (4Hz sampling), which is worth some power savings.