fs.writeFileSync() not working with serial link

Ahh - with RS485 you'd have trouble with CTS flow control. I'd suggest implementing a command & response sort of thing then... What you're doing won't work that well on any Espruino device: you get data from the serial port a few bytes at a time, and each time you're opening and closing a file... it's just taking too much time.

If you don't want to do some command/response thing you could use E.openFile to open the file and keep it open, which would help - and could also write in blocks of 512 bytes - which would match the block size in FAT