Cannot see backup drive...

[MMM]

Using the latest update. All of a sudden my backup drive does not show up under the backup drive drop down list.

It does however show up under the source drop down list.

Here is what it says in the log file;

Error | Unable to remove ACLs for / due to underlying error Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted" UserInfo={NSLocalizedDescription=Failed to clear permissions from file security object /, NSLocalizedFailureReason=Operation not permitted}

My computer sees the drive fine.

What should I be looking for to fix this issue? Thanks.