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I have a Windows XP Pro SP3 host machine running a Windows XP Pro SP3 guest machine. The host machine has one serial port, COM1, that I need to use in the guest machine.

When I add the serial port to the guest machine and try to start it, I get the following error:

Failed to start the virtual machine xxx

Cannot open host device 'COM1' for read/write access.
Check the permissions of that device (VERR_ACCESS_DENIED).

Unknown error creating VM (VERR_ACCESS_DENIED).

What are some of the things that might be causing this problem? How can I check "permissions" on a serial port?

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When you can not access a serial port in most of the cases something is using it already. If that is the case you can try to access the port with hyperteminal. If it can not open the port either, you just have to find what uses it, and shut it down.

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It is actually possible to not have permissions to open a serial port: try running as a limited user sometime! – SamB Jul 2 '10 at 22:43
Answer changed slightly. But I think it is still true in most of the cases, because most of the people don't use limited accounts on XP. – ytg Jul 8 '10 at 17:39
Using Windows 7. Was a "duh" moment when I realized I had kitty open behind virtualbox with the com port open... pretty easy fix: close kitty! – Jeremy Mar 18 '12 at 7:44

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