Problem with iMac waking up

[DP]

3.6 i5 16gb 10.6.8
Recently I have had a recurring issue with external HD's in that I sleep my iMac (although I will shut it down on average about every two weeks) and upon waking it up I have one or two of my externals won't show up on the desktop and I get the notice about not ejecting them properly. I never ejected them, just put the iMac to sleep. Also, they don't pop up either. I have to unplug and plug them back in for them to come back up on the desktop.
I have three externals and a TM-if this is a clue, I have a Seagate 750 FW and a WD Time Machine that don't disappear. The others are a 1TB Porsche Design USB (not sure of the manufacturer; got it from MacMall) and a 1TB WD USB.
A sign of problems? I have no problems or wonkiness or any other indication of anything wrong and the truant HD's work fine otherwise.

[Black]

Most important info missing:
How are the peripherals connected to the mac-- directly? Hub?
You've listed 2 external USB drives and two Firewire (assuming 800 all the way? No adapters?)
Which of these are the ones that don't reappear and need to be unplugged?

[DP]

Black wrote:
Most important info missing:
How are the peripherals connected to the mac-- directly? Hub?
You've listed 2 external USB drives and two Firewire (assuming 800 all the way? No adapters?)
Which of these are the ones that don't reappear and need to be unplugged?

Sorry, wasn't clear I guess. The WD Porsche Design 1TB and the Samsung 1TB are the ones that disappear and they are both USB 2.0 connected to a powered D-Link hub. The TM is a WD 500GB USB connected directly to the computer and the Seagate 750GB is the FW.
And that just lit up the light bulb in my head. The hub could be acting up although sometimes just one of drives won't show up. I'll connect the drives directly to see if it's the hub.

[Black]

In my experience pretty much all hubs start to get flaky at some point, even "name brands."