Sonos won't play music from certain folders on NAS drive.

OK. I’m guessing (hoping) there’s some kind of programme that can be run to find, and maybe even shorten, track names? Please God.

There are plenty of Apps that can do this - but confirm if this is the issue first?

If you are not editing the library, there is no need to enable Schedule Music Update. There is some variability during this process that you cannot control. Are there any visible error messages? You can submit a diagnostic and have support check for error messages that are not visible.

If there is an error the indexer will terminate. This can result in some very difficult to pin down issues because you cannot control the order of file processing. The indexer simply asks the NAS for a list of files and processes them sequentially. If there is an error termination, some files will be missing. If the files missing today are your favorites, you’ll notice. Otherwise, you may not notice that file XYZ is missing because the processing order is different today. Tomorrow, some of your previously ‘missing’ favorites might be present.

We don’t have access to the full diagnostics, but you can sort of dig through things and investigate the possibility of an indexer early termination. Go to http://[IP address of a player]:1400/support/review. This will bring up a web page of players. Click on a [player] → Zone Player Info and check the IdxTrk field. This should be blank. If this is not blank, the indexer had an issue while it was working on file XYZ. This is not a fun job because one player will take charge of the process and you don’t know which one this will be. You’ll need to dig through all of the players looking for a non blank field -- that may not exist.

This looks like fun🤣. Useful information. Thank you.

It became noticeable when the majority of my David Bowie albums disappeared. Not a happy man.

If you want to fuss with things, you can exert a little control.

Split your library into segments as you specify the Library folders. For example:

Davie Bowie
Seasonal
Other dreck

The indexer will individually work through these folders. Depending on your Library size and complexity, you could run out of player RAM while executing the indexer on the full library. Breaking the library into segments reduces the RAM footprint while processing a given segment. This trick has worked well for a number of large Library users.

I already have my library split into about 8 folders (A-B, C-D, E-G, H-K etc). I seem to remember splitting them into approximate number of songs, maybe 3,000 per folder. Are you saying it might be useful to split them down further, maybe one letter per folder (A, B, C, D etc) so this may aid the indexing? Am I understanding this correctly?

Just my 2 cents.

I've got my music library (968 GB) on my Western Digital My Cloud Home NAS, with 64,893 songs in 6,360 folders. I rarely do a rescan, only when I've added new music files. No splitting or segmentation of library necessary. My tags are simple: an album of 10 songs gets tagged (by mp3tag) 01.flac 02.flac or 01.mp3 02.mp3 and so on. Other necessary information (artist, album and what have you) in the tags. Never had any problems.

It just goes to show that the Sonos system does what it should do. In my opinion something must have altered your configuration that previously worked just fine. You might consider submitting a diagnostics within 10 minutes after a failed scan.

Is each of these folders listed as a separate entry in your Library setup? If so, I doubt that further splits will help unless your Metadata is grossly large.

You could change the order in your Library setup. If there is a some difficult Metadata causing the fail, You can somewhat locate the tracks by noticing what is missing.

Unfortunately, this approach may not help with Bowie if the rough spot is in A-B.

A diagnostic can pinpoint the file.

So, today I removed the 8 folders which contained my music. Then I added the C-D folder. This is the folder which had no music in it. When I looked in the folder the music was there. I then added all the folders back in and all music was showing. I then updated the music index. And...no music in folder C-D again. 

Anyone got any ideas please?

Yes, submit a diagnostic and post it here.

SSonos support can look at it and tell you what the issue is.  As you add each folder and re-index, does the library gradually increase?  If so, it suggests the library is full if you are able to see the missing data when everything isn't added.   I don't THINK Sonos adds to the index in any particular order.