Correct. But Externalizable is an old interface. It is retained to maintain compatibility with very old Java code. New code should rarely, if ever, need it.
Serializable, and the related serialisation concepts, are the way forward, for new code.
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.
I don't think Externalizable has in any way become obsolete (of course, it has had only limited use from the beginning). transient merely controls whether a particular field is serialized or not, while implementing Externalizable allows complete customization of the serialization process. Although I've never had to use it, I don't see which new feature would have replaced it.
Serialisation can do everything Externalisation can do, can't it? There are numerous hooks into Serialisation, so you can make it do almost anything you like. For instance, you certainly can serialise transient fields - it's only default serialisation that doesn't do that.
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.
Externalization is a hook into serialization -- it was introduced as part of the Serialization architecture; it's not any kind of an older API. It's what you use if you want to be 100% responsible for the bits written for a given class.