Battery cut didn’t sort it. However
after much experimentation, I have sorted it, so sharing for others here.
Solution was to format the USB stick as NTFS rather than Fat32. I also experimented with exFat which didn’t sort it.
My 2014 F10 M5 now has no trouble remembering exactly which track was last playing and resumes mid-track.
Some other info;
1. My other half has a 2014 f30 320d with basic nav and the strange thing is that I worked out hers only supports FAT32. NTFS and exFAT lead to an ‘unsupported’ message.
2. My old 2015 m135i also had basic nav and seemed to prefer FAT32.
3. I tried updating the car’s software and while it needed and update, it did not fix the issue.
4. While the F10 will read any file system, only NTFS has no issues and reads nice and quick. FAT32 would read quickly but last played was forgotten. exFAT was noticeably slow to read and last played was forgotten.
5. See my first post about clearing the memory/cache of the past 4 USB sticks. It didn’t sort my issue.
In summary, I advise that if your F-model BMW has basic sat nav, use FAT32 format. If you have widescreen sat nav (professional?) then use NTFS.