When DbBackup attempts to overwrite an existing file, it does not delete the file before renaming the -partial file to the desired file name.
As a result, the rename fails and the -partial file persists.
When another attempt to backup the database to the same file is performed, an IOException is thrown when the -parital file is found and the exception incorrectly suggests that something or someone may be writing to the file.
The resolution to this is simple.
If overwrite is specified, delete the pre-existing file prior to renaming the -partial file.
If the delete fails or if the rename fails, an exception should be thrown.