Deleting the ._* files from a mounted volume kinda screwed some things up
The symptoms you describe are the signs of a corupted filesystem, which is the
usual fate of frequently used USB key.

Make a backup of your USB key,
and run an fsck or Disk Utility on your file-system.

I even advise you to completely check it with:

• full backup,

• full secure erase with one pass of 0 (to be sure to write on all blocks),

• make a new FS,

• recover from backup.

Your find doesn't cause any trouble, since -exec argument isn't evaluated by a shell, you dont have to protect it against evaluation.