1. Enabled root user via ssh:
sudo su –
synouser –setpw root your_new_root_password

2. Login as root user via ssh:
ssh root@nas_ip_address

3. Download and install the exfat-fuse package as the following code:

It seems DSM 5.0 is 32bit linux, and DSM 6.0 is 64bit version, please download different packages.

DSM 6.0
wget -P /tmp/ http://mirrors.kernel.org/ubuntu/pool/universe/f/fuse-exfat/exfat-fuse_1.2.3-1_amd64.deb
dpkg -x /tmp/exfat-fuse_1.2.3-1_amd64.deb /tmp/exfat-fuse/
cp /tmp/exfat-fuse/sbin/mount.exfat-fuse /usr/bin/

4. Create a Shared Folder in Control Panal, e.g. usbexfat in Volume1

5. Plug the usb device with exFAT format, and in ssh:
fdisk -l

find out the exFAT device like this:

DSM 6.0
Device     Boot Start     End Sectors  Size Id Type
/dev/sdu1         256 7823654 7823399  3.7G  7 HPFS/NTFS/exFAT

6. Mount the exFAT partition:
mount.exfat-fuse /dev/sdu1 /volume1/usbexfat -o nonempty

7. Then can read & write the exFAT partition in Shared Folder usbexfat.

8. Unmount the exFAT partition:
umount /volume1/usbexfat

9. Eject the usb device in Control Panel or ssh:
eject -F /dev/sdu1