The Essentia Storage Bus does not seem to be aware that essentia reservoirs are more than just different-looking, more expensive jars. A single essentia reservoir is able to contain up to 256 essentia, not necessarily all of the same type. However, a storage bus will only store up to 64 in a reservoir, and it won't store any of a type that the reservoir doesn't already contain, unless it's empty. Note that importing works fine; it's just exporting that doesn't.