I have the exact same machine and have had success in trouble shooting it to the point of reliability. Here goes:
1) Trash everything Toast related. Remove all the USB extensions and trash 'em.
2) Make sure you have the latest Apple USB drivers.
3) Trash the two Lacie USB drivers (I forget their names). Install the updated drivers which are only available with Silverlining (don't ask me why). I'll mail 'em to you if you need them. Just contact me.
4) Install Toast. I've had better success installing Toast while your drive is on. Toast recognizes what it needs to install this way.
5) Reboot. and use. I guarantee this method. It took me almost a year to find out that there were updated Lacie drivers. Their tech support sucks.
6) Never update to 4.1.2. It does not recognize the drive. It is not supported in this version. Instead, Upgrade to Toast 4.2 straight from 4. 4 will work reliably though. Again, I'll send you 4.2 update if necessary.
Listen, drop me a line if you need help. I lost many hours of sleep over this drive. On more than one occasion I stopped myself from throwing it off the 14th floor
