Just upgraded to Mac OS 10.4.11. Now, my Toast 6.1.1 cannot mount saved disk image from my backups. It displays "...Mac OS Error. Result code = -60008". Anyone with similar problem? Is the Mac OS the culprit? Or could it be a remote reason that my backup images are sitting in a external disk (connected via USB2.0)?

I can't think of any reason for a disc image not mounting. So what I suggest is downloading and applying the 10.4.11 combined system update (presuming you did the incremental 10.4.10 to 10.4.11 update). There are odd USB and Firewire issues that sometimes happen after incremental updates that are fixed by applying the combined system updater.