Sorry if this has been answered already, but I'm still having this problem with my 64 gb (Class 10) card:
It plays fine in browse mode, but when trying to index, it seems to throw the player into an endless loop. I only have about 70 albums, and each album is in a single-level folder. Some of these folders are artwork subfolders.
On a 16gb card, indexing works very quickly (under 30 seconds) with a subset of these albums copied from the 64 gb card.
To get the 64gb card to work, will I need to only have single-level folders and delete the artwork subfolders? If so, then I think I'll stick with <64 gb cards.
Related question: is it safe to swap sd card while the unit is turned on? Asking this because I bricked my Sansa Fuze this way and don't want this to happen to my X3, especially on my first day.