Try to mount a dmg file -- Input/Output error

Question

I have certainly searched thru postings, but I cannot quite find a solution.
[1] An external FireWire drive holds a lot of dmg files of application installers.
[2] Double-clicking some, not all, of them results in an Input/Output error.
[3] Toast is always able to mount them.
[4] When the error happens, an attempt to copy the dmg file onto the Desktop results in another error message "The Finder cannot complete the operation because some data in "<name>.dmg" could not be read or written. (Error code -365)". At this point, clicking on OK brings up a Copy window which hangs. The only way out is to Force-Quit the FInder.
What should I do to rectify these issues?

Answer 1

I appreciate your suggestions.
The external drive in question is OWC's Mercury Elite-AL Pro with FireWire 800 (2) and 400 (1) ports
It was purchased in February, 2008. Its original connection was thru FireWire 800 to my G5 Tower.
[1] Ports
      The G5 has only one FireWire 800 port, while the OWC enclosure has two.
      So, I used the other FireWire 800 port on the OWC.
      The same intermittent I/O error in mounting dmg files.
[2] Cables 
      I tried another FireWire 800 cable with either of the two FireWire 800 ports on the OWC.
      The same intermittent I/O error in mounting dmg files.
      So, I switched the connection to FireWire 400 between my G5 and the OWC.
      No I/O errors after trying to mount numerous dmg files.
[3] MacBook Pro
      I disconnected the OWC and connected it to a MacBook Pro's FireWire 800 port.
      No I/O errors after trying to mount numerous dmg files.
From [1], [2], and [3] above, can I deduce that something is wrong with the FireWire 800 port on my G5 Tower? I thought that both FireWire 800 and 400 ports on the G5 Tower were controlled by the same chip.
Please advise.