mount /dev/sda /mnt/usb (-t 指定なし) では mount: you must specify the filesystem type になります． mount -t vfat /dev/sda /mnt/usb では mount: wrong fs type, bad option, bad superblock on /dev/sda, missing codepage or other error In some cases useful info is found in syslog - try dmesg | tail or soで， dmesgには FAT: bogus number of reserved sectors VFS: Can't find a valid FAT filesystem on dev sda.です．
