BeagleBone Black has onboard memory that can be flashed with an image file containing the operating system, with a few operating systems to choose from the onboard EMMC is only 4g which is plenty of room for a lightweight or headless installation.
once you update and install the needed packages to get your system setup you will soon see that 4g will dwindles rather quickly, especially if you wish to install a web server and database, or perhaps you are simply collecting data from some sensors or something, well if you use a MicroSD card to store your data you will be sure not to run out of space.
Using the MicroSD slot as extra storage is rather simple, one could even swap them out from time to time to gather the collected data from them sensors or even simplify modifying the content of a web server without the need to SSH into the BeagleBone Black to do so.
1. Format MicroSD.
Before you get started you need to format your MicroSD card that will be used on your BeagleBone Black, it needs to be formatted to ext4 and make sure you remove any boot flags from it.
2. SSH Into BeagleBone Black.
Power up your device and log into it using SSH
$ ssh miles@192.168.0.10
3. Create Mount Point.
Create a new directory for the mount point of your MicroSD card. You may call this directory any thing you like, for this example we will call it ‘Data’.
$ sudo mkdir /media/Data
4. Edit fstab.
Add the auto mount entry to the fstab so that the MicroSD card will be auto mounted at boot or when inserted during operation.
$ sudo nano /etc/fstab
copy and paste this line at the end of your fstab file. Remember to change ‘Data’ to the name you used earlier. Use (CTRL+o) to save (CTRL+x) to close nano.
/dev/mmcblk0p1 /media/Data auto rw,user,auto,nofail 0 0
5. Reboot.
just to make sure that the changes take effect, you can reboot your BeagleBone Black before inserting your MicroSD card.
$ sudo reboot now
6. SSH into BeagleBone Black.
After the system reboots log back into it.
$ ssh miles@192.168.0.10
7. Insert MicroSD.
Now you can insert your MicroSD card into your BeagleBone Black. and it will be automatically mounted.
$ cd /media/Data
8. Permissions.
At this time your MicroSD card is mounted an accessible by root only, if you wish to make it accessible to your user you will need to change the owner of the mount point. Remember to use your username in place of ‘miles’ and if you used a different name for the mount point earlier you should change ‘Data’ to the name you used as well.
$ sudo chown -R miles:miles /mdiea/Data
Congratulations !! you are now using your MicroSD card as extra storage on your BeagleBone Black using Ubuntu Server.
Linux Propaganda 
what is the maximum size of the microsdcard?
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i have used MicroSD cards from 2gigs up to 128gigs so its up to you on what size you use
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how do i write lxde to external storage(micro sd)?
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I’m not really sure i understand your question but LXDE is a desktop that you add to a Linux OS, like Ubuntu or Debian, i would suggest you maybe install one of them and then you can install LXDE threw the terminal, On the Beaglebone Black i do not use them as desktop PC’s so i usually install Ubuntu Server headless and SSH into them from my workstation to setup what i need for the project i am working on, if you want to use it as a desktop maybe Puppy Linux would be best as it works well with the Beaglebone, I am guessing but LXDE maybe a bit heavy.
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