今天ubuntu突然不能自动识别U盘了，用lsusb是可以看到U盘的，于是尝试着用mount手动挂载，使用sudo fdisk -l可以看到存在的盘符设备

Disk /dev/sda: 120.0 GB, 120034123776 bytes
255 heads, 63 sectors/track, 14593 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x8a941a0f

Device Boot Start End Blocks Id System
/dev/sda1 * 1 1913 15366141 7 HPFS/NTFS
/dev/sda2 1914 11952 80638267+ f W95 Ext'd (LBA)
/dev/sda3 11953 12076 996030 82 Linux swap / Solaris
/dev/sda4 12077 14593 20217802+ 83 Linux
/dev/sda5 1914 5738 30724281 b W95 FAT32
/dev/sda6 5739 11952 49913923+ 83 Linux

Disk /dev/sdb: 1999 MB, 1999568384 bytes
32 heads, 63 sectors/track, 1937 cylinders
Units = cylinders of 2016 * 512 = 1032192 bytes
Disk identifier: 0x4094441b

Device Boot Start End Blocks Id System
/dev/sdb1 1 1937 1952464+ b W95 FAT32

Disk /dev/sdc: 1064 MB, 1064566784 bytes
255 heads, 63 sectors/track, 129 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x0d0c0b0a

Device Boot Start End Blocks Id System
/dev/sdc1 * 1 129 1036161 b W95 FAT32

， 然后sudo mkdir /media/usbdisk创建一个文件夹，就可以用sudo mount -t vfat -o iocharset=utf8,umask=000 /dev/sdb1 /media/usbdisk/挂载U盘了，一开始老师提示“mount: wrong fs type, bad option, bad superblock on /dev/sdc,
missing codepage or helper program, or other error
系统日志中可能有一些重要的信息，
试试用 dmesg | tail 之类的命令查看。”

使用dmesg | tail可看到老是找不到盘

dmesg |tail
[ 1827.723226] VFS: Can't find a valid FAT filesystem on dev sdc.
[ 2109.229588] FAT: invalid media value (0xa5)
[ 2109.229595] VFS: Can't find a valid FAT filesystem on dev sdc.
[ 2200.193661] FAT: invalid media value (0xa5)
[ 2200.193669] VFS: Can't find a valid FAT filesystem on dev sdc.
[ 2465.332572] FAT: invalid media value (0xa5)
[ 2465.332578] VFS: Can't find a valid FAT filesystem on dev sdc.
[ 2501.270833] FAT: Unrecognized mount option "isochaset=cp936" or missing value
[ 2517.218127] FAT: invalid media value (0xa5)
[ 2517.218135] VFS: Can't find a valid FAT filesystem on dev sdc.

原来是我把加载的盘弄错了，应该是/dev/sdb1而不是/dev/sdb,sdb只是一个总称，sdb1才是具体的设备，

卸载用sudo umout /media/usbdisk就可以了.