Normally all the disks are mounted during boot time.

If something goes wrong you can manually mount the disks if nothing else is pausing a problem.

The simplest steps are :

1. turn a terminal by using the key combination CTRL+ALT+F1 (no graphics mode) to go back to gaphics mode use CTRL+ALT+F7 or sometimes F8
2. login in as admin
3. once logged in enter sudo -s
4. then either mount your missing disk with : mount /syno for example. Give simply the name of the linux mount point (check the file /etc/fstab)
5. to be sure you can also mount also the disk by : mount -a. All définition in the /etc/fstab are mounted