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I'm using the free version of VirtualBox v4.2.16. I'm trying to share a directory between the guest system (Centos 6.4) and the host (Windows 7 Enterprise).

The last time I used Centos, about a month ago, folder sharing was configured and worked. Now it's not.

When I select Devices/Shared Folders, I get a dialog which warns me that Guest Additions are not installed. When I select Devices/Install Guest Additions, I get a dialog which warns me that it is "Unable to mount the CD/DVD image C:\Program Files\...\VBoxGuestAdditions.iso." It asks me whether I would "like to force mounting of this medium." Below the text are buttons for "Force Unmount" (inexplicably, not "Force Mount") and "Cancel."

The "Details" button displays this information:
I looked at the host's VirtualBox directory. VBoxGuestAdditions.iso is there, right where it should be. I don't have any theories about why VBox can't read it.

Any suggestions?

Shut down the guest and in the settings storage select to remove the additions from the CD/DVD drive. Start the guest and try to install again.

I shut down the guest per your instructions and removed VBoxGuestAdditions. Then I started the guest again and clicked Install Guest Additions. I got an error box that said, "Could not insert the VirtualBox Guest Additions installer CD image into the virtual machine CentOS, as the machine has no CD/DVD-ROM drives. Please add a drive using the storage page of the virtual machine settings dialog."

At this point things stopped making sense. I returned to the VMBox window, reopened the Settings dialog, and selected Storage. I wanted to re-insert VBoxGuestAdditions under the IDE controller (that's where it was before). I selected the IDE controller, but I could not click the "Add Attachment" icon because it was disabled. (All of the icons below the storage tree display were disabled.) It appears that having deleted the non-functional addition from the IDE controller, I'm not allowed to reinsert it.

The options were disabled because you did not shut the guest down.
I also suspect that you deleted the CD drive instead of removing the media from the drive as I instructed. There is a small icon on the right that you should have clicked and it says remove from drive. If you click on the left one it say remove attachment. So now you need to click on the IDE and select to add a CD device. Don't mount the guest additions though. Best to do that by clicking install guest additions in the device tab of the guest.

Thank you. It seems to be working now.