The only rules text I can find related to a general ritual's duration is from the 'Duration' description on page 299 of the Player's Handbook:
This entry shows how long a ritual's effects last after the completion of the ritual. The effects usually last longer than those of a power.
Tenser's Floating Disk has '24 hours' as its Duration.
Within the description for Tenser's Floating Disk, it says:
If you are more than 5 squares from the disk for 2 consecutive rounds, the disk disappears, dropping whatever it was carrying.
Let's assume the disk is within 5 of the caster when the caster dies.
I think the biggest question here is whether the corpse continues to qualify as "you" or not. I would tend to think that a corpse is not the person that it was when it was alive, so I would say that the disk is no longer within 5 squares of "you", as you either no longer exist, or are in the afterlife.
On the other hand, the disk might not know you're dead, and just try and stay within 5 of your body. There's also the argument that death can be temporary in DnD, so your corpse isn't much different than your body when you're unconscious.
The Corpse is You
If the corpse is still the caster, then the disk is within 5 of the caster, and it will last out its duration.
Furthermore, if the corpse is still the caster, then when the corpse moves the disk will try to follow.
If you attempt this, a vital question is what the caster's speed is. Is it still what it was when it was cast? Is it what it was when the caster died? Is it zero since the caster can't move anymore?
I don't have any idea, and I don't see much guidance from the rules. Jonathan Hobbs made a fairly convincing argument that in DnD-4E, death is essentially an advanced version of unconscious, which is a temporary condition, which doesn't change your speed while in effect. That seems pretty reasonable, so let's say the speed remains what it was when the caster was alive. In any case, if the disk still identifies the corpse as the caster, then it doesn't seem to be paying much attention to the current situation, and I would expect it to use the caster's speed at the time that the ritual was completed.
The other question is where exactly you are. If we're going with the temporary effect concept, then you'd still be where your corpse is.
So, in this case, yes you could totally hijack the disk, and it would follow the corpse using the corpse's speed from when it was alive.
The Corpse is Just a Corpse, and You're Dead
If the corpse is just a corpse, and no longer the caster, then the disk is no longer within 5 of the caster.
A this point, the ritual's description says the disk "moves with your base speed once per round until it is within 5 squares of you". How to interpret this could vary wildly. I say you no longer exist, therefore you don't have a speed, therefore the disk can't move even if it knows where to go. Even if you want to use the speed of the caster at the time the disk was conjured, or at the time the caster died, I still don't think it would know where to go.
So, since it can't rectify not being within 5 squares of the caster, after 2 rounds it would disappear. This would occur whether you moved the corpse or not, since it no longer cares about the corpse.
Therefore, in this case, no you could not hijack the disk.
It makes more sense to me that the disk would remain stationary and disappear after 2 rounds.
However, given the nature of death in DnD-4E, it might be more in line with the rules to say that the disk follows the corpse around for its full 24 hour duration.