I bought new external DVD burner Plextor PX-608CU for Dell Latitude D420 one week ago.
I've noticed strange behavior of this connection.
I was trying to boot D420 with bootable DVD, but when I switched on D420 It stay on for 2 seconds and then switched off. Plextor was connected to external 5V power.
Ok I switched on D420 without Plextor and everything was OK than I've connected Plextor again first into USB and than to external power and D420 has just switched off (not shut down) and I've got a big question mark
above my head what is going on because other USB devices (HDD, sticks, Pinnacle DVC130...) are working without problems.
With this combination is obviously something wrong.
So I've decided lets go to Inet and do searching. After more than one hour I didn't find anything for this issue.
I didn't give up so I've connected Plextor again but first to external power than to USB. Everything was Ok so I've disconnected Plextor and connected It again but first to USB than to external power. Again was Ok.
So logical conclusion was that this issue has something in with temperature of D420. So I've tried the same with cold notebook and It happened again doesn't matter which connection I've made first.
I've checked external PS (it supposed to be 5V/2A). Without load output voltage is 5,25V and with 2A load is 4,8V.
It supposed to be Ok, but when I've connected Plextor to both USB ports (instead of external PS) and notebook booted up doesn't matter if it was cold or not.
So logical conclusion was that the problem is lying in PS, so I've connected Plextor to Laboratory PS and set output voltage exactly to 5V and again everything was OK. Then I've set output voltage to 5.2V and notebook wouldn't boot up again.
I've repeated this several times just to be sure that this 0.2V is causing this problems.
Obviously Dell Latitude D420 is checking USB +V pin (5V) for over voltage because of protection.

And solution?
-decrease output voltage of external PS,
-put Schotky diode in Plextor to prevent that the voltage from DC input will come back to D420 throught USB data cable,
-put external USB Hub between Plextor USB and D420 (yes it works),
-or (hardest one) find what is checking this volage in D420 and try to do something around it.

I'll try last one first and than in case of no success first one.

I've also checked Plextor with other notebooks (HP ZD7020, IBM TP (4x series), Asus, HTC) and I didn't notice any probs.

Jaka57, you should know, that every laptop has overvoltage protection against 5v 3.3v and 2.5v SB voltages. thus, i can propose, that every time you connect your external drive to USB this protection works. so,i'll try to give you advice turn on external power BEFORE you connect the drive to laptop, and check results.

The real secret is to return the drive. We have three. They all work fine with all laptops, Dell, Gateway, and Thinkpad.
This is a known problem drive... which is why it was unloaded for $25 to $30 by a number of online vendors a couple of months ago. Some work. Some don't.

Thank you for replies.
I didn't know that this is problematic drive because I didn't find anything bad about It on internet. But anyhow....
Yes I've tried to connect drive before switched on laptop (and all other combinations as well) but the same happened.
And what I've done.
Because I didn't find who or what is checking this overvoltage on MB and further to drop voltage of external PS is not quite a good solution because in case that I need to use another external PS and I believe that every PS has let say more than 5% tolerance as in my case, I've put Schotky diode in 5V line of USB connector on Plextor.

Now I can connect Plextor to external PS even if It has let say 6V or more. I've checked this.

Good news. Can you describe for us how the Shotky Diode is inserted in the 5V line of a US connector. Photos? Soldering involved?

Have you bothered to notify the manufacturer?

No I haven't bothered to notify manufacturer.
Because of this I haven't warranty anymore but anyhow I will survive without this...
Ok I've made this on PCB which has PS and mini USB on it.
First You have to unsolder the right most pin (lift it from pcb) of USB connector (looking from backside of it) then You have to solder Diode between that pin and middle pin of SMD element (It is some kind of transistor, I believe so because I didn't find spec for SMD code written on It). Anode of Diode goes to USB and cathode goes to that pin and thats all.
Because of warranty the better and easiest solution is to put this Diode in USB connection cable (usually 5V is red wire).