Found a really handy article which shows how to write directly to an external USB drive from a Synology NAS DS212J that is formatted HFS+ (aka Mac OS extended).
The trick is to disable journaling on the drive, then terminal into the box and remount manually by switching off the readonly flag for the drive. Zarino certainly explains this well in this article.
Quick summary below of what you need to to
Firstly, make sure SSH is enabled in your disk station by logging in and going to Control Panel -> Terminal and selecting "Enable SSH server"
On your Mac, open a terminal session to your NAS via it's IP address.
ssh root@192.168.0.10
Enter you password when prompted.
The authenticity of host '192.168.0.10 (192.168.0.10)' can't be established.
RSA key fingerprint is 02:.....
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.0.10' (RSA) to the list of known hosts.
root@192.168.0.10's password:
BusyBox v1.16.1 (2015-01-07 14:59:24 CST) built-in shell (ash)
Enter 'help' for a list of built-in commands
Have a look at how the drive is mounted to get the info you need. /dev/sdq2 is what you will probably need for the hfsplus area of the disk.
NAS> mount
/dev/sdq1 on /volumeUSB1/usbshare1-1 type vfat (utf8,umask=000,shortname=mixed,uid=1024,gid=100,quiet)
/dev/sdq2 on /volumeUSB1/usbshare1-2 type hfsplus (ro,force,uid=1024,gid=100,umask=000)
configfs on /sys/kernel/config type configfs (0)
Notice to "ro" flag to show it is read only highlighted above? Now unmount the drive.
NAS> umount -f /dev/sdq2
And remount so it is writeable using the same info used when it was mounted automatically by the box.
NAS> mount -t hfsplus /dev/sdq2 /volumeUSB1/usbshare1-2
Finally, you can now see how it is no longer readonly by running mount again.
NAS> mount
/dev/sdq2 on /volumeUSB1/usbshare1-2 type hfsplus (0)
Want to log in without putting in your password? Credit to Zarino once again as he has a good little
article on how to do this along with some other useful information worth having a look at.
Enjoy