MythTV 0.22 introduces us to storage groups across the board. This is no doubt a great thing, no more creating shares for network access, no more worrying about running out of storage space. With that said, there is one drawback to the new storage groups, they break playback of ISO files and VIDEO_TS folders. For those of us who have our DVD collections ready to go in digital form this may be an issue. This issue has been fixed in MythTV 0.24 (but only for unencrypted ISOs), the following work around will help you with all other cases:
Pre-Requirements: You will need your ISO/VIDEO_TS files to live in their own directory, separate from the videos storage group. On Mythbuntu I'd recommend using /var/lib/mythtv/ISO, but you can put them wherever you want. Remember that this directory should be owned by mythtv:mythtv and the permissions should be 775.
On your frontend lets make our way into the proper settings page (this may be slightly different for you, based on your theme):
Utilities/Setup > Setup > Media Settings > Video Settings > General Settings
Now in "Directories that hold videos" (which will be blank by default), you will enter the location of your ISOs.
It is also worth noting that filling in this location will fix Bug 469583 in which you cannot rip a DVD until this directory is filled in.
Now as long as you keep your ISOs/VIDEO_TS folders in this directory they will play fine. You can still put all your other video files in the videos storage group and they will work fine.
For additional information on storage groups you can read MythVideo 0.22 Transition Guide.