Must run full job again after changing tape to disk backup

In BE15, I run the full job with autoloader tape backup once and successfully(without run diff job).Since there is problem with tape backup, then I changed the full and diff to disk storage backup, I cannot run diff without run the full job again. Please refer to attached screen. It is the same job, and I only changed the backup media, why I have to run the full job again? If so, when there is problem with diff job on tape backup, we have to run full again and then we can run diff job on disk storage. But in BE2010 R2, we don't have this problem. We can run full job on tape, and then change diff backup to disk backup. Thanks.