This is a repost of an archive I posted in 2015.
This method works for DSM 6.2 using Jun's loader 1.03b for me.
1) Enable SSH and ssh into your DiskStation
2) Become root ( sudo -i )
3) Make a mount point ( mkdir -p /tmp/mountMe )
4) cd into /dev ( cd /dev )
5) mount synoboot1 to your mount point ( mount -t vfat synoboot1 /tmp/mountMe )
6) Profit!
admin@DiskStation:~# sudo -i
root@DiskStation:~# mkdir -p /tmp/mountMe
root@DiskStation:~# cd /dev
root@DiskStation:/dev# mount -t vfat synoboot1 /tmp/mountMe
root@DiskStation:/dev# ls -l /tmp/mountMe
total 2554
-rwxr-xr-x 1 root root 2605216 Aug 1 10:40 bzImage
drwxr-xr-x 3 root root 2048 Aug 1 10:40 EFI
drwxr-xr-x 6 root root 2048 Aug 1 10:40 grub
-rwxr-xr-x 1 root root 103 Jul 3 15:09 GRUB_VER
-rwxr-xr-x 1 root root 225 Aug 1 10:40 info.txt
root@DiskStation:/dev#