I am having the exact same problem with additional stuff.
I downloaded files from a computer several years ago to an Iomega portable drive.
I have 7.6 software, but the code I got with it will not work.
I tried to contact Retrospect all day with absolutely no results.
I finally downloaded a 30 day trial of 7.7 just to get the files I need.
I try and use the Retrospect 7.7 and also get to a point where it is looking for rdc. or .rbf files but not .rbd files.
I am tearing my hair out. I am not a computer geek, but usually can get what I want.
PS I would like to get a workable code to open the software I got with my Iomega HD and also find out how to open the .rdb files.
Please, please, please help me.