This is the strangest hardware problem I've come across. I'm running a unibody mbp (June 2009) with an external WD 2.0TB Caviar Green drive mounted in a firewire 800 enclosure made by icydock. Lately, I've noticed that after an unknown amount of uptime, the speed of file transfers to the WD external drive slows down dramatically. I figured this out by watching my time machine transfers take a bit longer after awhile, and I verified this using aja system test and speedutilities. When I boot my mbp in the morning, file transfers and time machine backups to the external WD transfer at a rate of about 67-75MB/s. This is the speed the drive should always be working at since it's connected via firewire 800. Aja speedtest verifies the speed is as it should be as well. Then, after a couple hours of using my mbp, or even later in the day, I'll transfer a file to the external WD or run a time machine backup and find that the transfer will only run at a max of 29-35MB/s. This is exactly half the speed the transfer should be going at. Subsequent transfers stay at this slower rate no matter what I do. However, I've been able to fix this problem by unmounting and remounting the external WD or restarting my mbp. After doing either of these actions, the WD external goes back to contant transfers at 75-80Mbs. So something is happening where the longer the mbp is running, or after some sort of disk activity, the external WD reverts to a slower transfer speed (closer to FW400). I can't isolate whether it's an issue on the mac os side or the hard drive or the hard drive enclosure because I can make the hard drive go back to optimal speed again merely by unmounting and remounting it. This works consistently until a few hours later when the drive begins to handle transfers at half speed again. I run hourly time machine backups to the drive. The time machine backups can't be the culprit because the drive remains running at optimal speed after the backups (verified with aja system test and speed utilities) Any possible ideas as to why the drive cuts down to half its optimal transfer rate after a bit of time? I can't figure out the cause of this, and the consistency of the drop leads me to think this is an issue with the power management on the drive since it reverts to full fw800 speed when I umount and remount the drive or restart my computer. Could this be an OSX issue where for some reason the system isn't negotiating the fastest speed? Any help appreciated.