This is probably because of the fundametal problem with the current
boot system in Debian, and the fact that the Linux kernel have become
event based and it is not possible to know in advance when a device
will be available during boot. It is basicly a race condition during
boot because the kernel return from kernel module loadin before the
bus handled by a kernel module is scanned.
To solve it, I believe we need to move to a event based boot system.
See <URL: http://lists.debian.org/debian-devel-announce/2009/09/msg00003.html >
for more information.
I am sure you find similar bug reports against the initscripts
package, even if it can't be solved by there without a structural
change in the way we handle boots, fsck and mounts.
Ubuntu uses upstart to get an event based boot system, and this is the
reason your external USB disk is mounted correctly there.
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