Ok, so the drives are mapped to network addresses (\\server\music\xtc.... etc.) on both the client and the server. So in the library, they are never referred to by their physical drive letter. This got round having to keep the drive letters aligned as the library grew and more and larger drives were added. I'm guess I've had it this way since maybe MC17, and it has worked perfectly, up until the point when I upgraded from MC20 to MC21 a few weeks back.
The ISO files are treated no differently from other video files, and are on the same drives, within the same logical structure as other video formats. So the library will be pointing to \\server\movies 1\c\casablanca and the folder may comtain an avi, mkv, Video_ts folder, etc or an iso. MC21 is behaving perfectly normally for all file types on a drive EXCEPT ISO. If will happily load and play movies, music stuff etc etc, in any other format, but, when asked to play an iso, on the same network drive, addressed in exactly the same way, it starts to misbehave as described below.
Bearing in mind that I'm primarily a theatre view user, and only use normal view for maintenance, my first view of the problem was in theatre view, and when asked to play a title in the library that happened to be an ISO, the following occurs; after pressing "watch", rotating MC cursor icon appears, happily circling. after a few seconds, windows task bar appears. Cursor continues to circle. I've never left it to see if it completes, but after a few minutes any attempt to stop, go back or do anything causes the standard "media center has stopped" popup to appear.
Looking at the same scenario from normal view, when you select an ISO file to play, an MC progress window appears, saying 'Downloading "internet resource"'. This is showing download progress, and the size of the download matches the size of the ISO. I've never left it to complete, as this would be a very long time. If I press "cancel" on the progress window, there is a pause, followed by an AnyDVD pop-up saying the disk is being analyzed and will be available shortly. This disappears and I get an MC pop-up saying the optical disk is being analyzed. This closes, and the ISO begins to play.
So, can I play the ISO? Yes, if I'm prepared to work in normal view, and then override a download. But I watch stuff though theatre view, and the other users in the house don't even know that normal view exists. But more importantly, this was all working perfectly in MC20, and I suspect several versions before that.
I havn't moved the files, I havnt changed the network (it's wired, gigabit). I just did a bog standard MC20/MC21 update. I've backed up the library and reloaded it, I've reinstalled both MC and underlying components like VCD. I've even restored an old image of one of the clients and re-installed.
Bearing in mind that I spent my entire working life in IT, the first 10 years of it as a programmer, I'm used to trying to debug stuff. Going back to earlier suggestions, it looks like something to do with the "play local version of files" parameter on the client. But I've checked that parameter, over and over again, and the system behaves in exactly the same way whichever way that parameter is set on the client.
I've been re-running the scenarios in the background while typing this, just in case I've missed something stupid, but it's still behaving consistently in the same way. The only thing I can think of to try is to re-image the server and rebuild the library, but it's a BIG library, and I've got custom fields with data I don't want to lose. I've re-installed MC and restored the library already. I've built a new test library. All have the same problem.
Don't get me wrong here, I love the product, and I have enormous respect for the skills of the development team, and the experience of the user base. I try not to post stuff on interact until I've exhausted my own (considerably smaller) knowledge and trawled for solutions in the wiki and interact. I'm sorry if I've come across as pushy or irritated, I'm just trying to get my system back up and running.
Thanks for the help so far.