Why does df output reports Input/output error.

Issue

• df command shows Input/output error for specific mount point.
• dmesg output

.FAT: FAT read failed (blocknr 6)
scsi 838:0:0:0: rejecting I/O to dead device
FAT: FAT read failed (blocknr 6)

• df output

[root@server tmp]# df -h
Filesystem            Size  Used Avail Use% Mounted on
....
....
....
df: `/path/to/mountpoint': Input/output error