Search:

serialEvent() is called from yield(). It's not called from interrupt context.

I believe this problem does show that we need to improve Teensy's fork of SdFat to call yield() while it's spending...

Also, and hopefully this isn't too "obvious", the default Serial1 receive buffer is only 64 bytes. Unless you've used attachRts() and whatever is sending the data stops when RTS signals Teensy's...

I don't know why. I would like to investigate, but I need a reasonably small test program to reproduce the problem. I looked briefly as your ZIP file. It is 21 files with a total of 7758 lines. ...