Success with 64GB microSDHC. Thought I'd add my $0.02. Being afraid to reformat 64BD SDXC to fat32, I found a no-brand name 64GB microSDHC card for $39 on Ebay. Loaded it as is in Cowon J3. Cowon didn't see it. Inserted into card reader on Win7 64bit PC. Saw it as 500 MB SC card. Fortunately I have Easeus partition master, so I fired it up and wiped out the card and formated as Fat32. No problems & fast. Copied a few flac files to it and reinserted it into J3. J3 sees & plays the files perfectly! When J3 is connected to Win7 PC via usb, card is not seen. Will try with Ubuntu virtual machine tonight just for kicks.
As stated before, you cannot move files to or from the card when it is in the J3 and connected to PC via usb (card shows 0 GB size). Took the card back out and reinserted into PC and moved many, many folders with flacs to the card. (took several hours). Loaded the card back into the J3 and about 1/2 the folders are empty. WTF? It may be the card is flaky (again no-name, looks like Chinese quality). Will retry moving over large number of files, but I can confirm that a 64GB microSDHC can can be read by Cowon J3, but formatting as Fat32 and copying files to and from the card must happen in the PC.