For many people, there is a common desire to store Recorded TV content on a network drive. It's a great idea in theory, as any non-copy protected recorded tv show can be shared between multiple computers capable of playing back the dvr-ms file format that Media Center records in. In practice, it is not that simple.
Getting the Recorded TV shows to show up in Media Center under the Recorded TV section of My TV takes a little more work, due to share permissions. Basically, ehSched and ehRecvr, the two services that Media Center relies on, run under a different context than the user that you are logged in as... it uses the 'Local System' account in all released versions of Media Center. As a result, simply sticking the UNC path to the share location isn't enough. Why? By default, you set share permissions up for the user, but that doesn't help the 'Local System' account (so Media Center would not read the files and add it to your list).
In a recent blog posting, Aaron Stebner, a Program Manager on the Windows Media Center team at Microsoft refers to a list of instructions posted on thegreenbutton.com, a popular Media Center enthusiast site, that instructs a user as to how to properly set up the user permissions so that the Local System account can access your share with the Recorded TV.
Aaron also adds an interesting tidbit for users of Media Center in Vista.
The Windows Media Center services that manage recorded TV (ehSched and ehRecvr) run in the Network Service context in Windows Vista, whereas they ran in the Local System context in Windows XP Media Center Edition. Therefore, you will need to add Network Service permissions in addition to the permissions listed in the forum post to get things to work correctly within Windows Media Center.
Do make note of the caveats that Aaron lists in the blog entry... especially the points about supportability and security.