Here is a third beta test for Teensyduino 1.54.
Edit: Links removed. Please use 1.54-beta9
https://forum.pjrc.com/threads/67252-Teensyduino-1-54-Beta-9
Changes since Teensyduino 1.54-beta2:
Add File base class and FS.h header
Add SdFat-beta library
Replace SD library with wrapper which uses SdFat
SoftwareSerial improvements on Teensy 3.5, 3.6, 4.1
Edit: Links removed. Please use 1.54-beta9
https://forum.pjrc.com/threads/67252-Teensyduino-1-54-Beta-9
Changes since Teensyduino 1.54-beta2:
Add File base class and FS.h header
Add SdFat-beta library
Replace SD library with wrapper which uses SdFat
SoftwareSerial improvements on Teensy 3.5, 3.6, 4.1