We believe that your issue has been closed by the upload of
Version 0.13.5 of grml2usb from Michael Prokop <mika@grml.org>.
The explanation is attached below

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Hash: SHA1

Format: 1.8
Date: Mon, 18 Mar 2013 15:12:10 +0100
Source: grml2usb
Binary: grml2usb
Architecture: source amd64
Version: 0.13.5
Distribution: experimental
Urgency: low
Maintainer: Grml Team <team@grml.org>
Changed-By: Michael Prokop <mika@grml.org>
Description: 
 grml2usb   - install Grml system / ISO to usb device
Changes: 
 grml2usb (0.13.5) experimental; urgency=low
 .
   [ Evgeni Golov ]
   * [a17807b] fix detection of strange (xxxNpM) structures when finding
     the device for a partition [Closes: issue1244]
Checksums-Sha1: 
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 752a7f32cd135eacaee2a4d2b880d99613af07db 390636 grml2usb_0.13.5.tar.gz
 5c01447c9ed9ace387281a500f7b11dbf077453d 216146 grml2usb_0.13.5_amd64.deb
Checksums-Sha256: 
 60b1e71608e157a7d12ec78d6f3a8bf69906ebd3d136dc48b69c8c2d45d5ff06 1106 grml2usb_0.13.5.dsc
 9329e21c2544dbe2f0741afcfa3d2e2e9c14566f5dfd023fab56eb3614d9d8c4 390636 grml2usb_0.13.5.tar.gz
 d2727782974d2db909d3f61737e97b2aff927067306b22acf2a6b8584ba0eb46 216146 grml2usb_0.13.5_amd64.deb
Files: 
 25823eb9e8e3d7fa1e7e2737e621bd2e 1106 admin optional grml2usb_0.13.5.dsc
 c1d14f9329342f56f3a0f7a03c0be9d8 390636 admin optional grml2usb_0.13.5.tar.gz
 cf1009aba7ec925305c2a5e4f2875f7a 216146 admin optional grml2usb_0.13.5_amd64.deb

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Thanks to Evgeni this is fixed in git already:

  http://goo.gl/in8mw

Especially for testing purposes it would be handy to install Grml systems to loopback mounted file images.

This currently fails because of the way partitions are handled:

  % sudo grml2usb grml64-full_2013.02.iso /dev/loop0p1
  [...]
      check_boot_flag(device)
    File "/usr/sbin/grml2usb", line 320, in check_boot_flag
      with open(boot_dev, 'r') as image:
  IOError: [Errno 2] No such file or directory: '/dev/loop0p'

We should find a smarter way to identify partitions.