You don't mention which operating system you're on, and reformat instructions are specific to the OS. Also, I assume the OS is installed elsewhere, on another primary drive. If your OS is on this hard drive it's whole different kettle of fish.
It sounds like there is some corruption in your files, and the drive wants you to essentially erase them clean so it can clear up the mess and start anew. That is what is meant by reformat.
Please go to this link and attempt to follow the LaCie instructions to "Fix volume corruption"
If you have trouble with this, or don't understand how to go about data recovery, you will need to learn how to backup the entire content of the drive before reformatting it. In that case, I recommend you consider using a full image backup utility like Acronis True Image http://www.acronis.com/
or Symantec Ghost.
If you have current backups of the data stored on this drive, and understand how to work with them to restore the information to your hard drive, you can safely proceed with the reformat.
If this answers your question, please take a moment to rate this post. And by all means, if you're not certain about how to proceed, write back and I'll be happy to point you to some tutorials that can guide you through the process.