Since it is a hidden folder, you may be completely unaware of this. On the calculator the folder name reads ".Tra". The only way I know of fixing this is deleting it by using the file manager on the prizm itself.
Just thought this information may be useful.
![;)](/dot/public/style_emoticons/default/wink2.gif)
EDIT:
The folks at omnimaga have notified me of two other ways to bypass this.
1. Empty trash before ejecting drive.
OR
2. Use Shift+Delete.