Android 6 and SD cards

From	Daniel James <daniel@me.invalid>
Newsgroups	comp.mobile.android
Subject	Android 6 and SD cards
Date	2016-12-22 20:58 +0000
Organization	A noiseless patient Spider
Message-ID	<VA.00000c19.29efe80e@me.invalid> (permalink)

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Having got my hands on a phone that runs Android 6 "Marshmallow" (a Moto G4 
Play) I thought I'd play around and try to answer a question I've asked 
several times that nobody has been able to answer.

Background

Android 6 can view an SD in either of two ways. It can be "portable storage" 
or "internal storage".

If a card is used as portable storage Android uses it in pretty-much the 
same way as it has in earlier versions: it's used to store data in an 
ordinary filesystem in such a way that the card can be removed from the 
phone and used in another device (such as a PC).

If a card is used as internal storage it becomes an extension of the phone's 
internal flash storage, and data that are stored on it can only be accessed 
in the Android device that formatted the card. Formatting the card this way 
enables it to be used to store apps (which in Android 6 you can't store in 
portable storage) and configuration data. etc.

The thing that I hadn't been able to find out is: Can you partition the card 
so that it has one partition that can be used as internal storage and 
another partition that is portable and can be read on another device.

Findings

My initial findings are this isn't easy to do, if indeed you can do it at 
all. 

An SD card formatted as "portable storage" has a normal BIOS-style partition 
table, with a single Windows-style FAT32 or exFAT partition for the data.

What I've discovered about an SD card that is formatted as "internal 
storage" is that it has a GPT partition table and two partitions as follows:

One named android_meta with type:
19A710A2-B3CA-11E4-B026-10604B889DCF9BECF4E3-44ED-48AC-A831-50D3C0FD0539 

and one named android_expand with type:
193D1EA4-B3CA-11E4-B075-10604B889DCF 359AB3A6-F7B7-B687-0FC8-DB399A9BC232

I didn't try to find out what sort of filesystem these partitions hold, and 
I believe that in any case at least one of them is encrypted. 

Conclusion

My hope was that it would be possible to create an SD card with two 
partitions, one of which was an unencrypted FAT32 or exFAT partition for use 
as portable storage, and one of which was whatever was needed for internal 
storage, but what I've discovered is that Android doesn't even use the same 
partitioning system for these two types of card, so it seems pretty unlikely 
that we can have the best of both worlds.

There are good security reasons for preventing the reading and writing of 
data from/to the internal storage of the Android device when the card is 
inserted into another device, but it's a pity that Google haven't made it 
possible to use just part of the SD card for internal storage and the rest 
for portable storage. It's particularly lamentable on the G4 Play (I don't 
know about other Marshmallow devices) because the rather simple camera app 
doesn't provide any means to select the destination for photographs, so they 
all go into internal memory rather than onto the SD card if it is set up as 
portable storage. JUST what you probably don't want!

I hope this is of interest to someone.

-- 
Cheers,
 Daniel.

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Android 6 and SD cards Daniel James <daniel@me.invalid> - 2016-12-22 20:58 +0000
  Re: Android 6 and SD cards Roger Mills <watt.tyler@gmail.com> - 2016-12-22 21:34 +0000
  Re: Android 6 and SD cards Daniel James <daniel@me.invalid> - 2016-12-22 21:39 +0000
    Re: Android 6 and SD cards The Real Bev <bashley101@gmail.com> - 2016-12-22 21:45 -0800
      Re: Android 6 and SD cards tlvp <mPiOsUcB.EtLlLvEp@att.net> - 2016-12-23 12:09 -0500
        Re: Android 6 and SD cards tlvp <mPiOsUcB.EtLlLvEp@att.net> - 2016-12-24 11:51 -0500
          Re: Android 6 and SD cards The Real Bev <bashley101@gmail.com> - 2016-12-24 10:06 -0800
            Re: Android 6 and SD cards tlvp <mPiOsUcB.EtLlLvEp@att.net> - 2016-12-24 13:21 -0500
              Re: Android 6 and SD cards The Real Bev <bashley101@gmail.com> - 2016-12-24 11:54 -0800
                Re: Android 6 and SD cards tlvp <mPiOsUcB.EtLlLvEp@att.net> - 2016-12-25 00:01 -0500
                Re: Android 6 and SD cards The Real Bev <bashley101@gmail.com> - 2016-12-24 21:44 -0800
                Re: Android 6 and SD cards tlvp <mPiOsUcB.EtLlLvEp@att.net> - 2016-12-25 14:18 -0500
                Re: Android 6 and SD cards ANTant@zimage.com (Ant) - 2016-12-26 06:45 -0600
                Re: Android 6 and SD cards The Real Bev <bashley101@gmail.com> - 2016-12-26 08:22 -0800
                Re: Android 6 and SD cards ANTant@zimage.com (Ant) - 2016-12-27 03:58 -0600
                Re: Android 6 and SD cards Frank Slootweg <this@ddress.is.invalid> - 2016-12-26 18:50 +0000
      Re: Android 6 and SD cards Daniel James <daniel@me.invalid> - 2016-12-27 15:31 +0000
        Re: Android 6 and SD cards The Real Bev <bashley101@gmail.com> - 2016-12-27 09:04 -0800
          Re: Android 6 and SD cards "Carlos E. R." <robin_listas@invalid.es> - 2016-12-28 20:52 +0100
        Re: Android 6 and SD cards tlvp <mPiOsUcB.EtLlLvEp@att.net> - 2016-12-27 23:46 -0500
          Re: Android 6 and SD cards Daniel James <daniel@me.invalid> - 2016-12-28 10:51 +0000
  Re: Android 6 and SD cards tlvp <mPiOsUcB.EtLlLvEp@att.net> - 2016-12-22 20:14 -0500
  Re: Android 6 and SD cards The Real Bev <bashley101@gmail.com> - 2016-12-22 21:43 -0800
    Re: Android 6 and SD cards Daniel James <daniel@me.invalid> - 2016-12-27 15:31 +0000
      Re: Android 6 and SD cards The Real Bev <bashley101@gmail.com> - 2016-12-27 08:59 -0800
        Re: Android 6 and SD cards Daniel James <daniel@me.invalid> - 2016-12-28 10:51 +0000
          Re: Android 6 and SD cards AL <l452236747@invalid.com> - 2016-12-28 09:10 -0700
          Re: Android 6 and SD cards AL <l452236747@invalid.com> - 2016-12-29 07:47 -0700
            Re: Android 6 and SD cards nospam <nospam@nospam.invalid> - 2016-12-29 16:19 -0500
            Re: Android 6 and SD cards tlvp <mPiOsUcB.EtLlLvEp@att.net> - 2016-12-29 15:13 -0500
          Re: Android 6 and SD cards The Real Bev <bashley101@gmail.com> - 2016-12-28 11:20 -0800
            Re: Android 6 and SD cards The Real Bev <bashley101@gmail.com> - 2016-12-28 15:25 -0800
          Re: Android 6 and SD cards tlvp <mPiOsUcB.EtLlLvEp@att.net> - 2016-12-29 04:04 -0500
            Re: Android 6 and SD cards nospam <nospam@nospam.invalid> - 2016-12-29 12:02 -0500
            Re: Android 6 and SD cards tlvp <mPiOsUcB.EtLlLvEp@att.net> - 2016-12-29 15:06 -0500
          Re: Android 6 and SD cards "Carlos E. R." <robin_listas@invalid.es> - 2016-12-28 21:15 +0100
      Re: Android 6 and SD cards Piet <www.godfatherof.nl/@opt-in.invalid> - 2016-12-27 18:55 +0100
        Re: Android 6 and SD cards Daniel James <daniel@me.invalid> - 2016-12-28 10:51 +0000
      Re: Android 6 and SD cards "Carlos E. R." <robin_listas@invalid.es> - 2016-12-28 21:06 +0100

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