Some fresh news from this bug :

It seems as if the system was not telling HAL the size of sdb1.

18:05:19.024 [I] blockdev.c:874: block_add: sysfs_path=/sys/block/sdb/sdb1 dev=/dev/sdb1 is_part=1, parent=0x08100160
18:05:19.024 [I] blockdev.c:1448: Ignoring hotplug event - cannot read 'size'
18:05:19.024 [W] blockdev.c:1481: Not adding device object

Complete log :

18:05:18.966 [I] osspec.c:241: SEQNUM=2994, ACTION=add, SUBSYSTEM=block, DEVPATH=/sys/block/sdb, DEVNAME=/dev/sdb, IFINDEX=0
18:05:18.967 [I] osspec.c:966: hal_util_find_known_parent: '/sys/block/sdb'->'/sys/devices/pci0000:00/0000:00:1d.7/usb5/5-5/5-5:1.0/host9/target9:0:0/9:0:0:0'
18:05:18.967 [I] blockdev.c:874: block_add: sysfs_path=/sys/block/sdb dev=/dev/sdb is_part=0, parent=0x080ffc40
18:05:18.967 [I] blockdev.c:1254: parent_bus is scsi
18:05:18.967 [I] blockdev.c:502: Probing storage device /dev/sdb
woohoo
[6741]: 18:05:18.970 [D] probe-storage.c:155: Doing probe-storage for /dev/sdb (bus usb) (drive_type disk) (udi=/org/freedesktop/Hal/devices/temp/136) (--only-check-for-fs==0)
[6741]: 18:05:18.970 [D] probe-storage.c:407: Checking for file system on /dev/sdb
[6741]: 18:05:18.970 [D] probe-storage.c:413: Doing open ("/dev/sdb", O_RDONLY)
[6741]: 18:05:18.974 [D] probe-storage.c:421: Returned from open(2)
[6741]: 18:05:18.974 [D] probe-storage.c:437: look for existing partitions for sdb
[6741]: 18:05:18.974 [D] probe-storage.c:447: partition sdb1 found, skip probing for filesystem
[6741]: 18:05:18.978 [I] partutil.c:875: MSDOS partition table detected
18:05:18.980 [I] hald_dbus.c:1366: storage.removable.media_available -> True
18:05:18.980 [I] hald_dbus.c:1350: storage.removable.media_size -> 7843348480
18:05:18.980 [I] hald_dbus.c:1334: storage.partitioning_scheme -> mbr
18:05:18.981 [I] osspec.c:241: SEQNUM=3000, ACTION=add, SUBSYSTEM=kernel, DEVPATH=/sys/kernel/uids/65534, DEVNAME=, IFINDEX=0
18:05:18.981 [I] blockdev.c:387: entering; exit_type=0, return_code=0
18:05:18.987 [I] blockdev.c:141: Add callouts completed udi=/org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979
18:05:18.987 [D] device_store.c:516: adding 0x8100160 to (linux.sysfs_path,/sys/block/sdb)
18:05:18.987 [I] hald.c:108: Added device to GDL; udi=/org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979
18:05:19.024 [I] hald_runner.c:659: running_processes 0x80e6508, num = 4
18:05:19.024 [I] hald.c:120: Started addon hald-addon-storage for udi /org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979
18:05:19.024 [I] device.c:4505: add_dev: subsys=kernel sysfs_path=/sys/kernel/uids/65534 dev= parent_dev=0x00000000
18:05:19.024 [I] osspec.c:241: SEQNUM=2995, ACTION=add, SUBSYSTEM=block, DEVPATH=/sys/block/sdb/sdb1, DEVNAME=/dev/sdb1, IFINDEX=0
18:05:19.024 [I] osspec.c:966: hal_util_find_known_parent: '/sys/block/sdb/sdb1'->'/sys/block/sdb'
18:05:19.024 [I] blockdev.c:874: block_add: sysfs_path=/sys/block/sdb/sdb1 dev=/dev/sdb1 is_part=1, parent=0x08100160
18:05:19.024 [I] blockdev.c:1448: Ignoring hotplug event - cannot read 'size'
18:05:19.024 [W] blockdev.c:1481: Not adding device object
18:05:19.025 [D] hald_dbus.c:3294: udi=/org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979
18:05:19.026 [I] hald_dbus.c:3320: AddonIsReady on udi '/org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979'
18:05:19.027 [D] hald_dbus.c:3294: udi=/org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979
18:05:19.027 [E] device.c:1705: In hal_device_inc_num_ready_addons for udi=/org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979 but all addons are already ready!
18:05:19.027 [I] hald_dbus.c:3320: AddonIsReady on udi '/org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979'
[6745]: 18:05:19.027 [D] addon-storage.c:748: **************************************************
[6745]: 18:05:19.027 [D] addon-storage.c:749: Doing addon-storage for /dev/sdb (bus usb) (drive_type disk) (udi /org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979)
[6745]: 18:05:19.027 [D] addon-storage.c:750: **************************************************
18:05:19.029 [D] hald_dbus.c:3223: udi=/org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979
18:05:19.029 [I] hald_dbus.c:3248: Local connection 0x8101c58 to handle interface 'org.freedesktop.Hal.Device.Storage.Removable' on udi '/org/freedesktop/Hal/devices/storage_serial_Sony_WALKMAN_492477603979'