I don't know if this is a bug or not. I used to copy files by selecting the disk drive mode. After that I will safely remove the device under windows before disconnecting the usb cable.
The problem is sometimes after copying files to sdcard, the SD card will become read only under android. For example I cannot copy or create files using file explorer. When using camera app it says "Unable to save file to SD card due to insufficient file permissions". When using album it says "SD card unmounted or not present".
After the problem occurs I cannot access the SD card via disk drive mode anymore, unless I physically removed and reinsert back the SD card.
The strange thing is I can still access the SD card in PC via card reader with no problem. I can also view the existing files in the SD card under android (eg opening .apk in SD card)
I tried flashing the ROM for 2 times and with a different SD card. Also I formatted the SD using Panasonic SD formatter and partitioned properly. The problem still appear after several access under disk drive mode. Million thanks!!!