Cannot mount drive

Has anyone had problems mounting a drive after installing an external hard drive (this drive was not used for the Image. The image is on its own hard drive in the computer. Could it be that the external hard drive assigned a letter that Acronis is not aware of and there is a conflict. I have about 5 images on this hard drive in the computer. It seems that some work, but if the image includs a hidden partician, which I can see to back up, will not be able to be viewed in Acronis Explorer.
Any help would be great.