Extremely slow FireWire on iMac 24" (Early 2008)

Question

Up until recently, my external FW800 drive (NewerTech MiniStack v3) has worked flawlessly with my iMac.  Very fast transfer speeds.  However, that is no longer the case.  The drive spontaneously unmounts, and file transfers on both FW800 and FW400 are slower than a floppy disk.  No joke!  Checking Activity Monitor, the read/write speeds are all over the place from 0 bytes to 40MB/second and anywhere in between.  While attempting a file transfer, the iMac becomes completely unresponsive when trying to use any program.  This was not a problem in the past.  The iMac was very responsive even with file transfers taking place in the background.  Now it is impossible to work on the iMac until the painfully slow file transfer is complete.  I think this is in part due to the halted disk activity from both drives.  If I connect the external drive to my Power Mac G4, it works flawlessly on both FW400 and FW800.  Both drives check out fine with Disk Utility.  Any idea as to why the FW400/800 ports on the iMac are acting so strangely?  I reformatted and reinstalled Snow Leopard thinking it might be a corrupted file, but the problem still exists.  Any suggestions?  Do I have a potential motherboard failure?  If so, the iMac is still covered under AppleCare until June 2012.

Answer 1

Try a SMC Reset. Please follow the instructions carefully.
SMC RESET
Shut down the computer.
Unplug the computer's power cord and all peripherals.
Press and hold the power button for 5 seconds.
Release the power button.
Attach the computers power cable.
Press the power button to turn on the computer.
PRAM RESET
Shut down the computer.
Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.