SD Card Multiple partitions USB access

Question

I want to split my SD cart in 2 partitions: one for android, and one containing a bootable OS used from computers through USB or by inserting in card readers. I can't access the second partition from USB!

Specs:

• Phone: motorola defy+
• System: CynogenMod 10 (android 4.1.2)
• SD Card layout:
  1. Primary FAT32, 1Gb, intended for android only (apps data)
  2. Primary EXT4, 15Gb, invisible from android intended for computers (bootable linux system)

Current status:

• Everything works with a laptop SD card reader: I can mount and browse both FAT32 and EXT4 partiations. I can boot from the SD card and boot to the linux I installed on the ext partition.
• From the phone: android can mount and access the FAT32 partition, the ext4 partition is invisible just like I want.
• Problem from USB: When accessing the card in the phone from a computer through USB (android is configured as USB mass storage device), I can only access the FAT32 partition.

My question:

Android seems to be exposing only the first partition as a block device through USB. Therefore I can't access the second partition, or even see it with partitioning tools. Obviously, booting from it also doesn't work. Can I make Android to expose the full SD card as a block device through USB, so that all partitions are visible from a computer and I can boot on it.

Any help or pointers appreciated, thanks!

Answer 1

You may only be able to do it via it's Recovery by using the Mount USB Storage function. It is present in all custom recoveries. The plausible reason that you cannot access the EXT4 partition in android is because it might be mounted and used by Cyanogenmod. You might be able to forcefully unmount the EXT4 in the Recovery on the Mount menu and unchecking the corresponding mount, if the kernel supports it.