Hi Ian,

Over the weekend I took my research one step further and started checking the CAT 5/6 wiring of the system components. I noticed that some functioned on a 100Mb/s level and others on the 1000Mb/s level the switch provides. I also experienced data dropout on one of the TwinStores after changing and rechanging switches. I rewired the CAT-cable on this component and since then the problems I mentioned in my post with C:Mac didn't occur.
So my educated guess for now is that the C:Mac problem was sparked by a momentary data dropout due to unreliable wiring: the iMac loses contact with the system, starts looking for it and finds it again when datatransfer is reestablished. Since the problems never occurred when I hooked up a windows machine, I guess C:PC is better equipped to handle temporarily data dropout than C:Mac is?!?
Anyhow, C:Mac is stable now for about 24 hours and when it remains that way for the next 12, I consider the problem solved for now.