This error message implies an int return type for SdFile::write().
C:\Users\transistor\Documents\Arduino\libraries\SdFat/SdFile.h:43: error: conflicting return type specified for 'virtual int SdFile::write(uint8_t)'
See line 43 of this file https://github.com/greiman/SdFat/blob/master/SdFat/SdFile.h
size_t write(uint8_t b);The return type should be size_t.
The same is true for this error:
C:\Users\transistor\Documents\Arduino\libraries\SdFat/MinimumSerial.h:30: error: conflicting return type specified for 'virtual int MinimumSerial::write(uint8_t)'See line 30 of https://github.com/greiman/SdFat/blob/master/SdFat/MinimumSerial.h
size_t write(uint8_t b);
It's as if size_t is defined as int by a macro in another include file. I can't imagine that so there must be another explanation.
Check the above files in your library located here to see if they agree with the ones on GitHub.