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@Rony > The reason the old software is not removed is it allows you to install a second copy in parallel to ensure that the newer version doesn't have any unforeseen problems running on your computer. If you do, you simply fall back to the old version and continue working.
There are faults, but even Serato DJ has updates to fix things and improve functionality.
If you have any other technical issues, please file a support inquiry here and our rekordbox team will be happy to assist you in troubleshooting and resolving them for you.