doing sudo fsck /dev/sdb2, i found that issue is there.
fsck from util-linux 2.20.1
e2fsck 1.42 (29-Nov-2011)
PI_ROOT contains a file system with errors, check forced.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
PI_ROOT: 183881/236176 files (0.2% non-contiguous), 851069/943360 blocks
Yes, my device is powered by usb. i have connected my device to my TV USB output.
how to come out of this issue, if there is power failure, i may end in this issue again.
is there any permanent solution to this issue.
Thanks for for all of your kind help.