I had two customers that needed to exempt a couple of systems from a group policy that disables USB/CD-ROM access, but I ran into the same issue both times when trying to do so.

I added the user to the appropriate group to block the GPO, but when I logged into the user’s PC, the drives still said access denied. I figured the group policy had not applied, so I forced it to apply and then I had the user both log off and back on and also restart with no success on the policy applying.

I did some digging and discovered that there is a bug in Windows that affects the Portable Device Enumerator Service. I tried several things with that service (restarting, looking at other depenedent services, etc) but nothing worked. Microsoft had a Hotfix available, so I tried that and still got nothing. Finally, after some additional research, I ran across a KB article that recommended going into Disk Management, uninstalling the driver for the CD-Rom and then rescanning the disks to let it re-install. As soon as I did that, everything started working properly. 

Here is the KB article with the Hotfix, in case it happens to work for someone else down the road: https://support.microsoft.com/en-us/help/2738898/users-cannot-access-removable-devices-after-you-enable-and-then-disabl