I think what may be happening here is that vCD's placement engine finds a "better" solution by putting the disk on the other datastore: it suffices that the other datastore have more free space for this to be true. We currently (I believe but will verify) do not have a soft affinity constraint to tie the disk to its original datastore when feasible (i.e. when that datastore has enough free space to accommodate the disk delta requirement).
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