We believe that your issue has been closed by the upload of
Version 0.3.30 of grml-rebuildfstab from Michael Prokop <mika@grml.org>.
The explanation is attached below

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Format: 1.7
Date: Sat, 13 Oct 2007 15:59:16 +0200
Source: grml-rebuildfstab
Binary: grml-rebuildfstab
Architecture: source all
Version: 0.3.30
Distribution: unstable
Urgency: low
Maintainer: Michael Prokop <mika@grml.org>
Changed-By: Michael Prokop <mika@grml.org>
Description: 
 grml-rebuildfstab - fstab-rebuilder for grml
Changes: 
 grml-rebuildfstab (0.3.30) unstable; urgency=low
 .
   * Ignore already mounted devices. This prevents hanging
     rebuildfstab process when an NFS share is mounted but not
     available. [Closes: issue309]
Files: 
 e4ddc8011f798ebda9deaeeacd62e0ac 616 grml optional grml-rebuildfstab_0.3.30.dsc
 d9a90f7cd6df7fc23aab9c459a5b7df3 10011 grml optional grml-rebuildfstab_0.3.30.tar.gz
 b251d792aad29b808e33b4d9d28fe97e 11652 grml optional grml-rebuildfstab_0.3.30_all.deb

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If rebuildfstab is executed and an NFS is mounted but unavailable then 
rebuildfstab is hanging. I didn't locate the main reason for this (nfs isn't 
part of /proc/partitions) but we should make rebuildfstab aware of this and 
make sure really only one instance of rebuildfstab is executed at a certain 
time.