Here is a couple of Peopoly posts regarding SD card usage issues.

We have noticed that a high percentage of file read issues resulted from user deleting gcode file right before adding new gcode to the same SD card. We are still verifying the cause and for now, we recommend user not to delete files from SD card. If you need more space, just format the card using the SD format application here: https://www.sdcard.org/downloads/formatter_4/

We will follow up on the situation

It is best to save gcode to hard drive and copy directly to the sd card’s directory. Don’t delete or move any files. If you want to start a big print, reformat using SD formater and print.

We recommend use “print” as the directory name for keep your gcodes. Do not keep gcodes in root directory as Moai will not be able to find it.

Use a name other than “gcode” for your directory and do not nested subdirectories. We use “print” in our lab and has worked pretty well.