Hello, lately we changed the object format of subject class from variableByte to normal one. All projects which using NBExternalStructure can meet certain problems with it, and first which comes is that it may load code cleanly but in fact result in bogus variable subclasses of NBExternalStructure , while it can't because superclass has instance variable. (i suspecting classbuilder bug, but need to verify first). To address that, you need to manually go over all subclasses and double-check. Another thing: since now instances NBExternalStructure can also point to external memory, special care should be taken for #copying. Sending #copy to instance which uses external memory, will create a copy on object memory (and copy all data from external memory into it). This is same behavior as in NBExternalArray. - there is a bugs in #initialize and #postCopy (fixed but not intergated yet) https://pharo.fogbugz.com/default.asp?11395#83020 will do that in nearest time. also note, that in callouts, you cannot pass an instance of NBExternalStructure anymore to function, which uses raw pointer (like void*) anymore. For passing a pointer to structure data for callouts, use myStructure address idiom (or use right struct type, which is preferable). -- Best regards, Igor Stasenko.