When complied under core 3.0.1 it complies and runs but significant numbers of characters are dropped from attempt to write the logfile on the SD Card, resulting in a garbled logfile. As I mentioned, it works fine when complied with core 2.7.4. I'm using the standard SD library: SD.h, SD.open, etc.
The is plenty of memory remaining on both the SD Card and in memory for buffers.
If I write a simple program to write data to the SD Card, I have no issues. So the problem appears to be some interaction between devices or interrupts and the hardware that is cause by 3.0.1 and not caused by 2.7.4.
I would appreciate any suggestions about how to fix or debug the problem.