Drop rates are not affected by HM or NM.
Sorry for short answer, I had to run and do something. Anyways, drop rates are always the same. The only thing that changes with HM is you get lockpicks and tomes added into the available items. And chests in HM are more likely to give you gold items.
Grrr, and that had nothing to do with your question. Purple (uncommon rarity) Crystalline Swords drop in either mode as they are only available in purple from Sorrows, UW or FoW chests.
http://wiki.guildwars.com/wiki/Crystalline_Sword