@JasonSkye Thanks for sharing your workflow. It is a little intimidating if I am honest, but then again, all your tracks can be adjusted within the hardware. I have a large collection of MP3’s that I have had for more than a decade that I just added to Rekordbox. Could this be the potential source of the problem maybe? Is it recommended to run MP3 files through platinum notes?
I hadn’t considered trying that. I did try it…and they were still locked.
Sorry for the super long list of workflow steps, I couldn’t think of a way of explaining it without posting the whole thing 
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Off-topic:
I think I’ll move and rename this topic to the right place if no one objects.
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Thank you.
@Fuzzywuzzymonkey I see you are in good hands here regarding the issues that you’re having so I see no point trying to add anything helpful but I just wanted to comment how nice it is to have a new user come on the forum and do things the right way:
Hope you will get all the info you need and if not resolve the issue then find a good workaround, cheers!
Thanks. I am here asking for free help, so I have no problem following sensible guidance. Plus, whilst looking for my answer on the forums, I did find some excellent examples of how not to ask for help!
I have to say everyone seems very welcoming and I have had amazing support, which is very much appreciated.
I have also made some progress in troubleshooting the locked beat grid issue. I was looking through @JasonSkye’s workflow considering what I could and couldn’t do and realised there was one thing that was very different to what I had been doing. When I first got the Mixstream Pro I exported my full Rekordbox library via XML into Engine DJ. I then used the Engine DJ Sync function to create a usb stick for use with the Mixstream Pro. @JasonSkye did not have this step and appears to go straight from Rekordbox into the hardware (which I hadn’t even considered trying). So, I grabbed a spare usb stick and just added a few playlists to it within Rekordbox (dragging and dropping via the side menu) and plugged that straight into the Mixstream Pro. Hey presto, the tracks that were previously locked were now fine and all beat grids could be amended, plus they carried over their tweaks so were all in sync first time. There must be an issue somewhere between how Engine DJ reads certain amended beat grids in the Rekordbox XML file that locks the beat grids in the hardware version of Engine OS.
I added a few more larger folders of music to the usb via the same method in Rekordbox (approximately 6000 tracks), but when I plugged it back into the Mixstream Pro, it did take a couple of minutes to fully read the usb. When selecting a particular track to play, it did take a few seconds for the waveform to fully appear, so not the same slick user experience as using the native Engine DJ Sync function.
I am interested to know what approach other people take. Do you use Rekordbox to create usb’s, the Engine DJ sync function or a combination of both?
I’ve found with importing from Serato that it’s still better to analyse tracks again in Engine, and I’ve never found the cue import to be that reliable so I end up just redoing them as I’m mixing (I don’t really do preparation as I sit in front of a computer for 8hrs a day anyway so don’t want to spend any more time). The import tool is a good way to move between software with playlists etc but not so good at importing the other stuff.
As far as levels go, I’d always advise somebody to do a manual check of levels in their headphones and using the level meters regardless of software, it’s the only surefire way of guaranteeing a smooth mix.
Ah yes the main downside to going straight from a Rekordbox USB into the controller is having to load each track individually to get track length and waveforms to display. Thankfully this only has to be done once, though it’s a bit of a pain. If you have loads of tracks to go through it can be easier to add them all to a single playlist and set that playlist to continuous play on one of the decks and leave it running overnight.
Glad the beat grid editing is sorted 
Sorry for late response, it got a bit busy at my side. Well, I usually import the rekorbox data base when needed (luckily not very often) to engine dj, then do all adjustments to beat grids and hot cues in Engine and then export to usb. Unfortunately I didn’t came across the same issues You have, so I can’t be much of a help. But as I see more users came in with options and solutions, so hopefully together you can solve the issue. Good luck.
I’m using the sycn tool Lexicon for Serato, RB and Engine. So all my libarys are in sync. Works fine with Beatgrids, Cues and Smartlists
I have just looked up Lexicon, interesting tool. I am also learning that people use other tools such as Platinum Notes and Mixed in Key to name but a few. I naively thought I could just use the one piece of software provided by the hardware manufacturer…but you live and learn!
As I have a working solution for now, I am going to resist trying any third party tools and see how I get on (partly because of the cost involved in one-off purchases and subscriptions). Thanks for the suggestion though.
I just wanted to say thank you for everyone’s assistance with resolving my issue. I was beginning to look at the Mixstream Pro with increasing frustration and more than a little buyers remorse. Now, I am back up and running and very much enjoying DJ’ing again. I couldn’t have done that in a couple of days without this forum.