What is filesystem panic?

[Per Anton Rønning]

I guess I should rephrase my question about my 4GB memory stick/pen, 
where the filesystem
now is set to readonly when I enter it into one of the slots.
Could anyone tell me what might have happened by taking a look at the 
snip of the logfile?

I am not allowed to do anyting but reading files. I am not allowed to 
delete files ,remove directires
or write to files, even if I change to root privileges.
I cannot say when this started - it just happened one time when I tried 
to backup some files to the Jet Flash.
Then I got the message (one example):
  cp: cannot create regular file `/media/disk/trade/statQ': Read-only 
file system
Does anyone have a clue? I'd be grateful for pointers in the right 
direction.

*** snip ***
Oct 27 11:36:13 localhost kernel: usb 1-2: New USB device strings: 
Mfr=1, Product=2, SerialNumber=3
Oct 27 11:36:13 localhost kernel: usb 1-2: Product: Mass Storage Device
Oct 27 11:36:13 localhost kernel: usb 1-2: Manufacturer: JetFlash
Oct 27 11:36:13 localhost kernel: usb 1-2: SerialNumber: TCC95547
Oct 27 11:36:18 localhost kernel: scsi 9:0:0:0: Direct-Access     
JetFlash TS4GJF185        8.07 PQ: 0 ANSI: 2
:
:
Oct 27 11:36:18 localhost kernel: sdb: sdb1
Oct 27 11:36:18 localhost kernel: sd 9:0:0:0: [sdb] Attached SCSI 
removable disk
Oct 27 11:36:18 localhost kernel: sd 9:0:0:0: Attached scsi generic sg2 
type 0
Oct 27 11:36:19 localhost hald: mounted /dev/sdb1 on behalf of uid 500
Oct 27 11:36:19 localhost gnome-keyring-daemon[2713]: adding removable 
location: volume_uuid_7A22_FF86 at /media/disk
Oct 27 11:36:30 localhost kernel: FAT: Filesystem panic (dev sdb1)   
[!!!!!!!! - my remark]
Oct 27 11:36:30 localhost kernel:    fat_free_clusters: deleting FAT 
entry beyond EOF  [My remark: What does this
                                                                        
                                                     mean? Looks like an 
error message]
Oct 27 11:36:30 localhost kernel:    File system has been set read-only 
[!!!!!!! - my remark]
*** end snip ***