What counts as ground for this ritual, if you have a 5x5 piece of stone/paper/metal/wood under the disk, and that is moved up, say 10 feet, would the disk phase through that stone, or hover 1 foot above it? What about floating over water?
I don't believe the rules ever formally state what exactly is or is not "ground." With that said, here's what I would use to evaluate this:
Does the item have a large enough surface area to support the disk?
Does the item have a reasonably uniform surface? (i.e. not someone's head and shoulders, not steeply angled, etc.).
How much weight can the item support? It doesn't necessarily have to support the full payload of the disk, but it should be more substantial than (for example) a cloud.
How much is the surface moving? Something that's moving a great deal (like the back of most conscious creatures) is probably not very "ground-like."
At the end of the day, it boils down to a GM call.
The Ground is the Battle Map.
While this answer will not satisfy the simulationists out there, it is probably the unspoken principle upon which D&D 4E, a combat-heavy system, is designed around. All the descriptions of various things that I've read talk about things that could be "on the ground" or are different from ground. For example, Tremorsense describes "walls or ground" which suggests that walls aren't ground. But if you're in a cave, what is the difference between a "wall" and the "ground" other than the relative position to the person asking the question? They're all "cubes" of the same contiguous mass of stone.
The fact is, D&D 4E never talks about "cubes" of terrain. Terrain is always portrayed in two dimensions, as squares. In other words, terrain is composed of the two dimensional features that adorn the two dimensional battle map. From this purely "gamist" perspective, Tenser's Floating Disk must always remain on a tile of the battle map and can never float off or above it.
Tenser's Floating Disk presumably ignores difficult terrain, as it is presumably a conjuration. It otherwise hovers over the ground, so it is likely that it would fall through any square of map terrain that a character with no special movement modes would fall through, such as water or a fake floor.
Beyond this, there may be a few other ways to think about three dimensional terrain to figure out what "Ground" is, keeping in mind that these are extrapolations from rules.
Ground is Blocking Terrain.
Although the compendium does not provide a definition for Ground, it does provide one for Blocking Terrain:
Walls, doors, large pillars, and various obstacles that fill squares on the battle grid are blocking terrain, which prevents movement.
Blocks Movement: Creatures can’t enter squares of blocking terrain. A typical square of blocking terrain is completely filled, which prevents diagonal movement across its corners.
No Line of Effect or Line of Sight: Blocking terrain blocks line of effect. It blocks line of sight as well, unless the terrain is transparent.
I think it's safe to assume that whatever Ground is, it would follow these traits. If the material in question were filling a square of terrain in front of the characters and the characters could not move horizontally through it, then that material could count as ground.
Water is not ground.
Water is not blocking terrain and creatures can enter water. Furthermore, the glossary entry for Underwater Combat states:
Underwater battles allow for up-and-down movement. Creatures can attack the characters from all directions, not just along the ground.
This suggests that squares of water are not Ground because you are moving in directions other than adjacent to the Ground.
"Weird" and floating terrain is ground.
There are precedents for blocking terrain that defy gravity, such as floating motes of stone or water in the Elemental Chaos. Furthermore, the Planeshaper Epic Destiny provides the following clue:
Fill 9 unoccupied squares with a solid surface, such as stone or wood. If you fill a square with a solid surface that is not attached to another surface (in other words, you create a stone slab 5 squares up in the air), the surface hovers in place.
As the power allows the character to fill squares with a solid surface, and filling a square with a solid is how Blocking Terrain is defined, this seems to suggest that the Planeshaper can create Ground that floats mid-air. Presumably, the Planeshaper could create a tile of ground mid-air, cast the Tenser's Floating Disk ritual, and continue to create a "path" mid-air and have the Disk follow behind.
Ground has no limit to its load-bearing properties.
Another possible way to think about ground is that it is a surface that can support infinite weight. I have never come across a reference to "falling through the ground" in D&D 4e, no matter how heavy something may be. Thus, one possible way to discern ground would be to see if there is a limit to the weight it can hold or if it is breakable.
Tenser's Floating Disk has a capacity limit of up to 2,000 pounds, so it probably isn't ground: you probably can't float one disk over another. However, Tenser's Lift has no load limit, so you probably can float a disk on a lift and use the lift to move the disk vertically.
I'd say the ground under the disk does need to be able to support the disk - it's meant to be read as an "effectively infinite" source of upward force.
But, I'd say it doesn't need to be uniform or such. It's essentially a hovercart - it can detect the force it's applying to things on it, apply the necessary counterforce to keep them from falling off, and magically support itself on what's beneath it without the physical restriction of touching.
(Heh. Maybe it can never apply upto 2000 pounds of force, no matter how high your Arcana. Instead, what's increasing is its coefficient of friction. High level disks are sticky.)