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Devs answered some questions about legacy storage.
How do I get Legacy storage?
You will get Legacy storage for free with the completion of the "Introduction to Strongholds Mission" upon acquiring your first Stronghold. Legacy storage will also be craftable and is possible to appear on the Cartel Market. (In both cases this means it would be available on the GTN for credits as well)
If I add another Legacy storage to my Stronghold, does this add more storage space?
It does not. Adding more Legacy storage "units" will simply add another access point, similar to how the cargo hold works right now. You can increase the size by adding more bays (more in the next answer).
How big will it be?
Legacy storage comes, by default, with one bay unlocked. That bay will have 80 slots in it. You will be able to purchase additional bays with credits or Cartel Coins.
Can I transfer Bind on Pickup (BoP) items through my Legacy Storage?
This is still being worked on/decided in development. We should have an answer on this soon™.
My questions is if all alts can use crafting supplies stored in legacy storage. That would be sweet.
Also I hope that there will be more cargo bays than we have on ship.
Sith Warrior - Story of Hate and Love http://www.youtube.com/watch?v=sxKrlwXt7Ao
Imperial Agent - Rise of Cipher Nine http://www.youtube.com/watch?v=OBBj3eJWBvU&feature=youtu.be
Imperial Agent - Hunt for the Eagle Part 1http://www.youtube.com/watch?v=UQqjYYU128E