I have been checking a fews things on the pcmanfm 0.97 and udev and mounting removable media and so on, and finally I noticed that in /etc/udev/knoppix-post.rules is running /usr/sbin/rebuildfstab in background, ie at the back of the command, there is a '&'.
I am not sure of the original intent of that, but since rebuildfstab can be a slow process, it can happen that when pcmanfm is displaying the plugged in usb drive, the directory /media/sdb1 is not created yet but the /etc/fstab is built already ( ??? - that seems to be the case on my computer ). When user click on the drive, they get 'directory not found' thing. But if you wait long enough, the directory /media/sdb1 is created.
My system is not representative of all other systems, as I have already obsoleted 'hal' and already using pcmanfm 2 but nevertheless I notice more consistent behaviour after I remove the '&' and the end of the knoppix-post.rules.
Anyone what to share some observation of this on his computer ?
DELL PowerEdge R730XD Server 2x E5-2680v3 2.5GHz =24 Cores 32GB H730 4xRJ45
$267.00
Dell PowerEdge R630 Server 2x E5-2680 V4 = 28 Cores S130 32GB RAM NEW 480GB SSD
$197.99
Dell PowerEdge R630 Server 2x E5-2640v3 2.60Ghz 16-Core 64GB H330
$182.65
Dell Poweredge R630 Server 2x E5-2620 V4 =16 Cores | S130 | 32GB RAM | 2x trays
$159.99
Dell Poweredge R730xd 12 Bay LFF 2x SFF 2x E5-2680v3 2.5ghz H730p No Ram No HDD
$219.99
Dell PowerEdge R720xd 26HDD 300gb 2.5-inch E5-2697 X 2CPU 384RAM 7.2 Tb HDD 
$180.00
Dell PowerEdge R620 Server 2x E5-2660 v2 2.2GHz 20 Cores 256GB RAM 1x 480GB SSD
$139.99
Dell Poweredge R730xd LFF 14-Bay 2U Server | Choose Your CPU & RAM Config
$489.99
DELL PowerEdge R430 8SFF 2x E5-2680v4 2.4GHz =28 Cores 32GB H730 4xRJ45
$226.00
DELL PowerEdge R730 Server 2x E5-2680v3 2.5GHz =24 Cores 128GB H730 4xRJ45
$345.00