I have heard the argument that the disk simply has no propulsion mechanism beyond the caster's movement, only moving in relation to the caster. In order for it to be functional as an improvised mount, one must accept the notion that the caster can sacrifice his own would-be movement in order to have the Disk move instead. Arguments for this base themselves on it taking a move-action to direct a spell-effect, and the "unless otherwise directed" clause.

I personally think there are ways to get mount-ish use out of the disk without having to accept that the caster can actively direct the disk's movement. Any method that could be used to propel oneself forward while under the effect of the levitation spell ought to work, as the disk levitates and thusly does not exert any form of pressure on the ground beneath while one sits on it. The cheapest non-magical method I can think of is using some sort of paddle. For simple magical means, an Unseen Servant ought to do (limited speed though).

Another potential method, dependent on how detailed the "otherwise directed" directions can be, is to direct the disk to maintain its position relative to a specific part of the caster's body, rather than the whole caster. If the disk is directed to stay centred under the caster's left hand, then the caster sitting on the disk could make the disk move forward by simply extending his hand over the edge of the 3-foot disk. The disk would then move at its maximum speed to try and centre itself under the palm of his left hand, moving the caster, thusly creating indefinite propulsion in the direction of the hand. Think the cartoon cliche of getting someone to move by dangling food in front of them with a fishing rod.