Is it OK to plug in new hardware when the computer is on?

When I was using my laptop, a technician told me not to plug in the mouse, a keyboard, or any new hardware when the computer was on.  Is this true for desktops as well?  I have a printer without an on/off button - I have to plug the cord in to turn it on and I always do it after I turn my computer on.  Thanks!
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Are you talking about devices connected via USB ?

I have had bad experience just plugging in the USB device without Stopping the device when the computer is ON.
I would sincerely advise you not just by experience but also seeing several cases to

a) Stop the device using the green arrow in the system tray

b) shut down or make the computer to standby mode and then remove the device

Lee W, MVPTechnology and Business Process AdvisorCommented:
NEVER, on your typically home computer, open the case and plug things into the circuit boards or cables connected to the circuit boards while the computer is one.

Things that connect to it while the case if ON (like USB devices, Firewire Devices, Printers, Monitors, etc) can be safely attached while the computer is on.  PS/2 keyboards and mice (circular connectors with 6 pins and a plastic peice) are the only exceptions I'd make as (A LONG time ago) I did see doing so blow out the connector on the main board.  However, in the years since, I've done that often enough myself and NOT seen a problem.

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LucynkaAuthor Commented:
Thank you.  Yes, my printer is connected via a USB cable, but I plug in the power cord, not the USB cable, to turn it on.
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LucynkaAuthor Commented:
I also turn on my external hard drive by plugging the power cord in.  And my DVD drive.
With newer computers (within the past 5 years or so) you should be fine plugging in not only USB but the PS2 while the PC is on.  Asleew said, anything inside the case should never be touched with the power on.
Depending on your operating system and specific hardware (i'm talking about mice and keyboards here specifically) you can actually plug them into the PS2 port while the computer is running and they will run fine.  This is acceptible on Win XP, however I havn't had much luck in 98 or ME.  I think Win 2000 does, but I am not sure.
The 'remove device' is important to try to remember to use, especially on Win 2000.  If you have a flash memory drive and simply pull it out in Win 2000 you'll most likely loose the data you recently transferred, or in the worse case corrupt the flash drive.  Fortionately win XP is more forgiving in this area.
LucynkaAuthor Commented:
Thanks, that's helpful.
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