一般来说，接上U盘后，Linux内核是可以识别到U盘的，但Android层需要进行挂载该U盘，才能被读取到。在init.rc中看到一个网址，于是便打开来查看，原来Android有相应的方法来挂载U盘的。摘录如下。

本文摘自https://source.android.com/devices/storage/config-example.html

Android 5.x and earlier

Physical primary only

This is a typical configuration for a device with single external storagedevice which is a physical SD card, like Nexus One.

The raw physical device must first be mounted under/mnt/media_rw where only the system and FUSE daemon can accessit.vold will then manage thefuse_sdcard0 servicewhen media is inserted/removed.

fstab.hardware

[physical device node] auto vfat defaults voldmanaged=sdcard0:auto,noemulatedsd

init.hardware.rc

on init

mkdir /mnt/media_rw/sdcard0 0700 media_rw media_rw

mkdir /storage/sdcard0 0700 root root

export EXTERNAL_STORAGE /storage/sdcard0

service fuse_sdcard0 /system/bin/sdcard -u 1023 -g 1023 -d /mnt/media_rw/sdcard0 /storage/sdcard0

class late_start

disabled

storage_list.xml

android:mountPoint="/storage/sdcard0"

android:storageDescription="@string/storage_sd_card"

android:removable="true"

android:primary="true"

android:maxFileSize="4096" />

Emulated primary only

This is a typical configuration for a device with single external storagedevice which is backed by internal storage on the device, like Nexus 4.

init.hardware.rc

on init

mkdir /mnt/shell/emulated 0700 shell shell

mkdir /storage/emulated 0555 root root

export EXTERNAL_STORAGE /storage/emulated/legacy

export EMULATED_STORAGE_SOURCE /mnt/shell/emulated

export EMULATED_STORAGE_TARGET /storage/emulated

on fs

setprop ro.crypto.fuse_sdcard true

service sdcard /system/bin/sdcard -u 1023 -g 1023 -l /data/media /mnt/shell/emulated

class late_start

storage_list.xml

android:storageDescription="@string/storage_internal"

android:emulated="true"

android:mtpReserve="100" />

Emulated primary, physical secondary

This is a typical configuration for a device with multiple externalstorage devices, where the primary device is backed by internal storageon the device, and where the secondary device is a physical SD card, like Xoom.

The raw physical device must first be mounted under/mnt/media_rw where only the system and FUSE daemon canaccess it.vold will then manage thefuse_sdcard1service when media is inserted/removed.

fstab.hardware

[physical device node] auto vfat defaults voldmanaged=sdcard1:auto

init.hardware.rc

on init

mkdir /mnt/shell/emulated 0700 shell shell

mkdir /storage/emulated 0555 root root

mkdir /mnt/media_rw/sdcard1 0700 media_rw media_rw

mkdir /storage/sdcard1 0700 root root

export EXTERNAL_STORAGE /storage/emulated/legacy

export EMULATED_STORAGE_SOURCE /mnt/shell/emulated

export EMULATED_STORAGE_TARGET /storage/emulated

export SECONDARY_STORAGE /storage/sdcard1

on fs

setprop ro.crypto.fuse_sdcard true

service sdcard /system/bin/sdcard -u 1023 -g 1023 -l /data/media /mnt/shell/emulated

class late_start

service fuse_sdcard1 /system/bin/sdcard -u 1023 -g 1023 -w 1023 -d /mnt/media_rw/sdcard1 /storage/sdcard1

class late_start

disabled

storage_list.xml

android:storageDescription="@string/storage_internal"

android:emulated="true"

android:mtpReserve="100" />

android:mountPoint="/storage/sdcard1"

android:storageDescription="@string/storage_sd_card"

android:removable="true"

android:maxFileSize="4096" />

Android 6.0

Physical primary only

This is a typical configuration for a device with single external storagedevice which is a physical SD card, like the original Android One. There is nosecondary shared storage and the device cannot support multi-user.

init.device.rc

on init

# By default, primary storage is physical

setprop ro.vold.primary_physical 1

fstab.device

/devices/platform/mtk-msdc.1/mmc_host* auto auto defaults voldmanaged=sdcard0:auto,encryptable=userdata,noemulatedsd

Emulated primary only

This is a typical configuration for a device with single external storagedevice which is backed by internal storage on the device, like Nexus 6.

Primary shared storage (/sdcard) is emulated on top of internal storage.

No secondary SD card storage.

USB OTG storage devices supported.

Supports multi-user.

fstab.device

/devices/*/xhci-hcd.0.auto/usb* auto auto defaults voldmanaged=usb:auto

Emulated primary, physical secondary

This is a typical configuration for a device with multiple external storagedevices, where the primary device is backed by internal storage on the device,and where the secondary device is a physical SD card, like Xoom.

Primary shared storage (/sdcard) is emulated on top of internal storage.

Secondary storage is a physical SD card slot that can be adopted.

Supports multi-user.

/devices/platform/mtk-msdc.1/mmc_host* auto auto defaults voldmanaged=sdcard1:auto,encryptable=userdata