我正在插入 USB 记忆棒，但并未安装并且有点不可见。
$ sudo lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 178.9G 0 disk
├─sda1 8:1 0 256M 0 part /boot
├─sda2 8:2 0 50G 0 part /
└─sda3 8:3 0 128.6G 0 part /home
$ dmesg
[159171.366480] usb 1-3: new high-speed USB device number 29 using xhci_hcd
$ ls -al /dev/sd*
brw-rw---- 1 root disk 8, 0 Sep 15 10:48 /dev/sda
brw-rw---- 1 root disk 8, 1 Sep 15 10:48 /dev/sda1
brw-rw---- 1 root disk 8, 2 Sep 15 10:48 /dev/sda2
brw-rw---- 1 root disk 8, 3 Sep 15 10:48 /dev/sda3
这是怎么回事？