For an XT (class) machine you'd have to stick to well known hard disk geometries, e.g. 615 cylinders, 4 surfaces, 17 sectors per track.
If your harddisk from the AT driver is 20MB but doesn't conform to some standard that was around at the XT's time it won't be recognised.
One way to find out (unless you know) is to plug the harddisk into one of the AT drivers that have a BIOS that can auto-detect the harddisk. It will tell you the geometry it recognizes.
As for the 3.3 boot disk, the 3.3 DS/DD set included the boot disk per se and an utilities disk, make sure you boot from the correct one.