I had freezes about once an hour running on an external with no disc in drive. The last two times I played (roughly 3 hours total) I had no freezes at all. All I did was rename the folder from "BLES01555 - [Ni No Kuni - Curse of the White Witch]" to "BLES01555"
It actually seems to transition from battle to world map a little quicker now (i rarely see the loading animation any more and I used to wait 4-5 seconds), and the book opens in about 15 seconds. I don't know how slow that was before since I hadn't opened it.
Idk if this actually fixed anything, but I haven't had a freeze since. It sounds too simple, so I'm sure its not a complete fix, but it does seem like its a read speed problem.