I've found this error reported before, but not any clear resolution. You may want to try these two previously offered possible resolutions:
1) Make sure Windows Explorer is not open when unmounting.
2) Mount to a different drive designation.
You can accomplish the second option by clicking the Options button before entering the passphrase for mounting the virtual disk. If you are using NTFS, another related option would be to mount to a folder instead of a drive designation (this requires mounting to an existing folder that does not contain any files).