Growing a single disk into a 2 disk raid0

[Phillip Susi <psusi@cfl.rr.com>]

I had someone wanting to take an existing disk full of data and expand
it by adding a second disk.  This seemed reasonable, and I thought of
several ways you could do this, but none of them pan out:

1)  Create a single disk raid0 using the existing disk, then reshape it
adding the second disk.  mdadm fails to add the second disk as a spare
so that you can reshape, with the kernel complaining that personality
does not support diskops.

2)  Create a single disk raid1 using the existing disk, then reshape it
adding the second disk.  Apparently you can not reshape from raid1 to raid0.

3)  Create a raid10 using the existing disk, setting the number of near
copies to only 1 so as to preserve the existing data, then reshape
adding the second disk.  I ran into two problems here:

a) mdadm refuses to create a single disk raid10, saying that at least
two devices are needed for raid level 4 or 5.  I am surprised that you
can not create a single disk raid10 even with --force, and the message
mentions the wrong level.  I would think that a single disk raid10 with
2 far copies would be fairly useful.

b) I tried 2 disks with one missing and mdadm refuses to create a raid10
with only a single near copy.  The kernel complains that layout 0x101 is
unsupported.

Is there any way to accomplish this?