You might be over the 65K track limit or variable meta-data limit.

Data is hidden from users so try a reindex and then when it errors contact Sonos to have them look at the internal data to see if they can tell you what is wrong.

File size is not an issue. Unless you have also crossed the track limit or metadata limit, 500MB doesn’t matter to SONOS. I will note that SONOS has little patience. If the NAS is too slow to startup, SONOS will abort the attempt.

Thank you Stanley_4 and buzz to take time to answer.

The thing that puzzles me the most is that I managed to establish the same Music Library under Sonos S1. But not under the S2 generation. 

I’m not an expert with these matters, but I think that I’m not near the 65K track limit with my 500 MB Music Library (for testing purpose).  The file size of my Music Library I want uploading to Sonos is around 10GB.

The size of Meta data I guess will be reflected within the space requested and displayed of the entire file. And as I see it Meta data will be included in every music file - MP3 files. But I really don’t know about this stuff.

Again I think that the NAS device is awake and responding quickly enough as I successfully produces the smaller files.  

I don’t know if Sonos is doing some kind of preparation - like scanning the directory of the Music Library before starting the indexing process, and then fails within a specific time. But on the other hand Sonos is warning that the indexing could take up several minutes. When trying with the original file Sonos is failing within 10 seconds.

So, I’m stuck with this one.

Have a nice day

Flemming Sigvardt, Denmark 

Which model NAS are you using?

The NAS I’m using is WD MY BOOK LIVE.

There are some WD topics here if you go searching.

I tossed my My Book Live after it kept frustrating me.

Ditto. If the drive has some years on it, I think that it is a lot less frustrating to replace it with something else.