Almost everyone that was using Time Machine with Mavericks and now converted to Yosemite is having the same problems accessing their information in the manner that they were used to. It is not a hard drive problem. The problem is with the software from Apple. They need to fix it.I am using a WD desktop "MY BOOK" drive. Also you need to understand something, all of us are backing up to our designated HD, it is just that you can no longer go into TM from the icon and access the different downloads from the right hand RED bars. If you notice, when doing this, the bars will be dull in color indicating-no access.
The work around that others found and posted back in October is to go to Finder, select your TM drive, then select users, the name of your computer or your name, then select the category that your save would be located , example downloads, documents, etc. , search and copy what you need to your desktop. Its just the process of loops that you have to jump thru to get what you want.
Apple has taken us back to the days of Windows XP!! Ha, but not so funny!