Thanks for your reply, gibor. Yes, it may be something like that, although my fstab in Slitaz3 doesn't even specify any USB devices ...
# [Device] [Mount Point] [Filesystem] [Options] [dump] [fsck order]
proc /proc proc defaults 0 0
sysfs /sys sysfs defaults 0 0
devpts /dev/pts devpts defaults 0 0
tmpfs /dev/shm tmpfs defaults 0 0
/dev/hda1 /media/seagate ext3 root,rw,noauto 0 0
/dev/cdrom /media/cdrom iso9660 user,ro,noauto 0 0
But who knows what's going on when an external USB drive is plugged in? It had never occurred to me before but I just ran a simple fdisk and found that I can actually see all 4 partitions of the external USB drive ...
root@linux:/home/tux# fdisk -l
Disk /dev/hda: 1082 MB, 1082460160 bytes
14 heads, 10 sectors/track, 15101 cylinders
Units = cylinders of 140 * 512 = 71680 bytes
Device Boot Start End Blocks Id System
/dev/hda1 59 15039 1048576 83 Linux
Note: sector size is 4096 (not 512)
Disk /dev/sdd: 3000.5 GB, 3000558944256 bytes
255 heads, 63 sectors/track, 45599 cylinders
Units = cylinders of 16065 * 4096 = 65802240 bytes
Device Boot Start End Blocks Id System
/dev/sdd1 1 11400 732557312 7 HPFS/NTFS
/dev/sdd2 11400 22800 732557312 7 HPFS/NTFS
/dev/sdd3 22800 34200 732557312 83 Linux
/dev/sdd4 34200 45600 732560384 83 Linux
root@linux:/home/tux#
... hda1 is my infamous 1GB Seagate drive and sdd1-4 are the four partitions of the 3TB WD USB drive. But what ever I try I cannot mount partitions 3 and 4, which is probably the reason why neither PCMan nor GParted even show them.
But I do not want to create too much fuss about the whole thing as everything works fine in Slitaz5.