/etc/fstab mounting with auto buffer flushing

My /etc/fstab line for the SD I'm willing to mount looks like:

/dev/mmcblk1    /media/sd_card  vfat    user,rw,umask=000,sync  0 2

With this line I'm able to mount the SD, but everytime I want to unplug the SD I have to execute blockdev --flushbufs /dev/mmcblk1 if I want to make the written changes take effect. What means, that if I dont execute the blockdev-command all the written changes are lost after unplugging the SD. The weird thing is, that I all the time I was using this line, I thought everything is working fine... So what am I doing wrong? ---------- I'm working on the jetson tx2 with Ubuntu 16.04 running on it.