I am clearing off all my music from BT Cloud because I have it backed up elsewhere. So far in two days of progress I have cleared off nearly 200GB of files, early 45,000+ files in total and there's still 93GB of space in use, most of which is still music.
The reason there is so much space in use is due to the fact that most files have duplicates. some files have as many as 7-8 copies cluttering up the space! Most of these files are tracks ripped from CDs and so would never need to be updated or altered in any way, so why so many copies?
This sort of question is often best resolved by thinking of the process underlying the system. In Backup, the app scans through the diectories of files to be backed up and any file not currently in the Backup is copied to the 'cloud' (or whatever file system). For each file, the metadata (creation, modification, etc date, size, etc) are saved to a (hidden) database. As the app scans, each file is checked against the database. A file not listed in the DB is copied and metadata added to the DB. If the file is listed in the DB, the metadata of the file currently in the backup and the source file are compared. If they are the same, the file is skipped. If they are different, the app has to decide whether to replace the existing backup file with the one it has found - generally yes if it is newer. Actually not replaced - the existing backup file is marked as an old version and retained in the backup (at least in BT Cloud)
With Sync rather than Backup, files are copied to the cloud from more than one source, and are written back to the sources always based on the metadata, not the contents of the file itself. In this case, comparing the correct metadata fields is critical, as well as correct handling of conflicts (see my thread 'Can we see the light at the end of the tunnel?').
You do not say whether you are using Backup or Sync, but as you are dealing with music files the main metadata should not be subject to any change (assuming you are only listening, not editing). Some fields, such as Access Date would change, but (I submit) these should be disregarded by the app.
Perhaps your observation that files that should not be superceded are being may help to identify what is going wrong with BT Cloud, as in the incorrect handling of conflicts.
Hope this helps...