If your firewire hard drive is failing intermittently or after transferring some data, it could be a heat issue. Symptoms might include i/o errors and spontaneous disconnects.
Simply removing the case (housing) from my Western Digital MyBook drive allowed it to work reliably enough to get all of the data off. I might continue to use it that way since it hasn’t failed since removing the housing.
I’ve seen this issue with LaCie cases as well. They get extremely hot and are prone to failure, especially if the case does not have a fan and the drive is used continuously for long periods of time, such for backups.
If you need to use a firewire drive for backup, I would invest in a good case with a fan if possible.