Xsane and rebooting scanner each time I resume work with the soft ?

From a newbie (two months on Ubuntu, what work !)

I scan with a Agfa Snapscan U (a usb scanner) with both Xsane and Vuescan depending on the nature of the work I have to do (each has its strengths and weaknesses photo/negatives). My scanner is permanently plugged and on a first session with Xsane, it is detected and works fine. Same with Vuescan. But if I close the soft and want to resume work later, then xsane won't detect again and I need to "replug" (English is not my native language) the scanner with the usb connector, upon which it will work fine. This scanner works perfectly well with Windows.

What puzzles me :

1/ the /etc/fstab file doesn't show the "none /proc/bus/usb usbfs defaults 0 0 that apparently should be there, though my usb connectors work initially (could this be a hotplug/cold plug issue ?)

# <file system> <mount point> <type> <options> <dump> <pass>
proc /proc proc defaults 0 0
# /dev/sda5
UUID=c0ed83aa-0293-4681-b797-0e449cca521c / ext3 defaults,errors=remount-ro 0 1
# /dev/sda7
UUID=62ede78b-95b0-4ddd-b408-58030d5fe68b /home ext3 defaults 0 2
# /dev/sda1
UUID=1156-13E8 /media/sda1 vfat defaults,utf8,umask=007,gid=46 0 0
# /dev/sda6
UUID=993d3370-88d9-461c-9a66-b86d1194efb9 none swap sw 0 0
# /dev/sdb1 /media/sdb1 vfat defaults,utf8,umask=007,gid=46, 0 1

/dev/scd0 /media/cdrom0 udf,iso9660 user,noauto 0 0
/dev/scd1 /media/cdrom1 udf,iso9660 user,noauto 0 0
/dev/fd0 /media/floppy0 auto rw,user,noauto 0 0

2/ the syslog excerpt (I figured those where the relevant lines) "talks" about ppdev. What is it and what is the relationship to usb accesses ? Did I miss something here ?

Jul 31 18:52:19 pascale-desktop gconfd (pascale-5628): démarrage (version 2.18.0.1), pid 5628 utilisateur « pascale »
Jul 31 18:52:20 pascale-desktop gconfd (pascale-5628): Adresse « xml:readonly:/etc/gconf/gconf.xml.mandatory » résolue vers une source de configuration en lecture seule à la position 0
Jul 31 18:52:20 pascale-desktop gconfd (pascale-5628): Adresse « xml:readwrite:/home/pascale/.gconf » résolue vers une source de configuration accessible en écriture à la position 1
Jul 31 18:52:20 pascale-desktop gconfd (pascale-5628): Adresse « xml:readonly:/etc/gconf/gconf.xml.defaults » résolue vers une source de configuration en lecture seule à la position 2
Jul 31 18:52:20 pascale-desktop gconfd (pascale-5628): Adresse « xml:readonly:/var/lib/gconf/debian.defaults » résolue vers une source de configuration en lecture seule à la position 3
Jul 31 18:52:20 pascale-desktop gconfd (pascale-5628): Adresse « xml:readonly:/var/lib/gconf/defaults » résolue vers une source de configuration en lecture seule à la position 4
Jul 31 18:52:26 pascale-desktop gconfd (pascale-5628): Adresse « xml:readwrite:/home/pascale/.gconf » résolue vers une source de configuration accessible en écriture à la position 0
Jul 31 18:52:29 pascale-desktop NetworkManager: <information>^IUpdating allowed wireless network lists.
Jul 31 18:52:29 pascale-desktop NetworkManager: <WARNING>^I nm_dbus_get_networks_cb (): error received: org.freedesktop.NetworkManagerInfo.NoNetworks - There are no wireless networks stored..
Jul 31 18:54:22 pascale-desktop kernel: [ 276.105078] ppdev0: registered pardevice
Jul 31 18:54:22 pascale-desktop kernel: [ 276.151707] ppdev0: negotiated back to compatibility mode because user-space forgot
Jul 31 18:54:22 pascale-desktop kernel: [ 276.152222] ppdev0: unregistered pardevice
Jul 31 18:55:22 pascale-desktop kernel: [ 335.504582] ppdev0: registered pardevice
Jul 31 18:55:22 pascale-desktop kernel: [ 335.527054] ppdev0: negotiated back to compatibility mode because user-space forgot

Finally, the KDE literature (and Sane man pages) say I should have libusb 0.1.6 installed though I could only find a 0.1.4 version with Synaptic when I installed libusb three weeks or so ago. Last of last, some usb scanners manufacturers indicate a 10-local rules with usb access 666 (and vendor ID) is needed and others say it is a 50-udev-default.rules. I have not done so yet, is it necessary ? Of course, I have changed usb access permissions in 45.libsane.rules from 644 to 666 and everywhere else that seemed necessary. I may have forgotten something.

Can this libusb /rules mess be untangled so I can use my scanner (photo is the reason why I need a computer) without re-plugging several times a day ? Obviously, there is a missing link somewhere.

Ah, last thing : I have a hp printer (old 930C) and I noticed it was in the scanner group. Could that conflict though it is on a LPT1 port ?

So many thanks for trying to work out these usb issues

With much hope...