Ran chkdsk /R /F from command prompt, and it found no errors. I also ran "error checking" (right click on drive, properties, tools, "error checking"). It, too, found no issues.
Some guys on another forum had problems with a circuit board in a particular batch of 1DX's and actually had to send it back to Canon to have the board replaced. This likely isn't it, but I'm just sharing this so that you are on the lookout in case it happens again, with other cards. Don't be afraid to send it in. Canon will not charge you.