Apple - Problem cloning the startup disk of a MacBook Pro running OS X 10.11.6 El Capitan

The system partition type of SSDs defaulted to CoreStorage in 10.11 (El Capitan).

The CoreStorage partition (usually disk0s2) is a container which can store one or more volumes. Only the innermost objects (logical volumes) export to additional device nodes. Further reading: CoreStorage.

If you asr --source ... a CS partition (in your case disk0s2) to a target partition, you will not get a proper bootable file system (e.g. a bootable HFS+ volume). The simple reason: a CS partition has no traditional file system and a different internal structure compared to an HFS+ boot volume.

Solution:

1. Instead of asr --sourcing a disk slice simply use the exported logical volume.

   There is no easy way to get the device node of the mounted logical volume of the SSD automatically (i.e. to use it in a shell script). diskutil list or diskutil cs list list it, but it's difficult to extract the device node with tools available in Recovery Mode (with e.g. awk ... or sed ...) - at least for me with limited shell scripting capability. The best I have found is bless --getBoot. The default boot volume has to be the internal SSD before booting to Recovery Mode (with the option key or cmd-R) - compellingly! You can also set the start volume to the internal SSD in Recovery Mode.

   On the command line (in Recovery Mode) the asr command would look like this then:

   CSB=$(bless --getBoot); asr restore --source $CSB --target target_device_node --erase
   

   If you get the error Source volume is read-write and cannot be unmounted after executing the asr ... command, try to unmount $CSB after defining the variable CSB: diskutil umount $CSB.

   You will get a bootable HFS+ volume on a type HFS+ partition on the target disk finally.

2. If method 1 fails you can also use the mountpoint of the SSD's system volume (e.g. Macintosh HD):

   asr restore --source /Volumes/Macintosh\ HD --target target_device_node --erase
   

---

I tried to asr blockcopy the CoreStorage source partition (disk0s2) to a target partition with the same size using different methods but all fail with a checksum error. These methods require to modify the partition type of the target partition with gpt afterwards.

Here I just put a re-worked version of the Basic Procedure section in the original question. Following the path sketched below solved my issue. You may regard this as a kind of preliminary exercise for a script-based solution. Fame on @klanomath if its useful for you, shame on me if it sounds like gibberish to you.

Basic Procedure

Terminology
source_disk_id
       disk identifier of the Macintosh HD partition

source_device_node
       device node corresponding to the Macintosh HD partition

target_disk_id
       disk identifier of the target partition on the external HD

target_device_node
       device node corresponding to the target partition

target_partition_size
       size of the target partition
       Note: Used on invocation of a command, take care to use an appropriate size specifier.

Processing Steps

1. Create the target partition that is to contain the bootable clone.

   1. Determine the size of the Macintosh HD partition via
      diskutil info source_disk_id.
   2. Determine the size of the Recovery HD the same way running diskutil info; usually yet another 650 MB.
   3. We estimate the target partition's size so that the target partition can hold the contents of the Recovery HD as well as that of the Macintosh HD, including free space. This is more or less a precaution to prevent asr restore, later on used, from complaining about missing space.
      When the cloning operation will be accomplished, the target partition's size may be reduced running diskutil resizeVolume.
   4. Now we are ready to create the target partition:
      diskutil resizeVolume target_disk_id target_partition_size JHFS+ FreePartition 0
      Note: This works for me because the target disk is maintained such that there is a "remainder partition" with respect to on-disk order. Running the diskutil resizeVolume command then simply cuts off a chunk of disk space from the remainder partition's upper end that shall now be used as the target partition.

2. Switch to Recovery Mode

   1. Determine the source_device_node
      When we have a look at the CoreStorage Logical Volume Group to which the Macintosh HDbelongs, we have to select the disk identifier of the associated Logical Volume - in contrast to the Physical Volume.
      Note: The partition type of the Physical Volume is Apple_CoreStorage while partition type of the Logical Volume is Apple_HFS, equipped with a JHFS+ file system.

      CoreStorage logical volume groups (1 found)
      |
      +-- Logical Volume Group 9344A028-DD9F-454C-89C0-8E2866E5FBB6
      -   ---------------------------------------------------------
      Name:         Macintosh HD
      Status:       Online
      Size:         250140434432 B (250.1 GB)
      Free Space:   8921088 B (8.9 MB)
      |
      +-< Physical Volume EC0BB005-738C-4F32-8B27-BA8801EBC34D
      |   ----------------------------------------------------
      |   Index:    0
      |   Disk:     disk0s2
      |   Status:   Online
      |   Size:     250140434432 B (250.1 GB)
      |
      +-> Logical Volume Family A20BC6DA-C477-44B4-82C9-C88B2CB41658
      ----------------------------------------------------------
      Encryption Type:         None
      |
      +-> Logical Volume 73C52081-F8CF-4C86-93F9-4BBA68602854
          ---------------------------------------------------
          Disk:                  disk2
          Status:                Online
          Size (Total):          249779191808 B (249.8 GB)
          Revertible:            Yes (no decryption required)
          LV Name:               Macintosh HD
          Volume Name:           Macintosh HD
          Content Hint:          Apple_HFS
      
      

      A somewhat more direct way to determine the boot volume`s device node is just to invoke bless --getBoot, provided it works in your environment.

      2. Now run

      asr restore --source source_device_node --target target_device_node --erase
      

      Invoked this way, asr restore will restore(clone) and verify both partitions, Macintosh HD as well as Recovery HD.

3. Back in Normal Mode, run diskutil rename to assign more meaningful names to the two partitions just "made" by asr restore, something like "my_mbp2015_macintosh_hd_osx10.11.6_yymmdd" and "my_mbp2015_recovery_hd_osx10.11.6_yymmdd" resp.