Image mounting issue

I am running the following: Acronis 2009 build 9809 On August 10, after doing a full new backup to an external Western Digital drive and attempting to mount it, I got this error:

Cannot assign a drive letter to a partition from the backup archive

 After much experimenting, I discovered that I can mount this partition in "read write" mode, but not in "read only "mode. I have other backups on this drive and I am able to successfully mount them in read only mode.

I redid the backup a number of times several times changing the computer configuration as follows:

 --disabled Norton antivirus and firewall while Acronis created the backup and during the attempt to mount the backup

--removed all software that was installed after the time that Acronis worked correctly

--rebooted the computer after all these changes in configuration

Acronis still failed to mount the backup in read only mode. I am running Acronis 2009 build 9809, Windows Vista Home Premium, SP 2.  Thank you for your help.