I thought I needed to add this (as a fix) to a problem that I was having lately.
I developed a small application to create some files on the SDCard. The application works fine on the Emulator (creates directories, files etc). However, when migrated to the real physical Blackberry 9000 device, I noticed a problem where Directories created ok, however, writing or even opening any files on the SDCard where giving an exception of
"File system error"
(example; writing to a file "file:///SDCard/BlackBerry/xyz/test.txt").
I searched the forums and the net all over the place, nothing was found
I even checked all application permissions, etc, and even added more code to my application to allow more permissions.
At the end, I figured it out!... it was a very silly problem
Aparently the SDCard was locked and inaccessable when connecting your BB device to the PC via USB. Once you plug in the USB cable into your BB device, you would be prompted for a password to allow the PC to access your BB drives, once you key in your BB's password, the PC then would "lock" all your drives on the blackberry causing the same "File System Error" with all applications that requires the SDCard to write to.
To make your application work, simply, unplug your USB cable and re-run your application !
I hope this helps.