Which version of rekordbox are you using, and is that device that you have connected formatted to FAT32 or HFS+?
Hi, I have the free version of Rekordbox, and I cannot find my sync manager. Normally it's on the bottom left, but now it's just disappeared. I've attached an image of how my Rekordbox looks. Any help would be greatly appreciated.

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Which version of rekordbox are you using, and is that device that you have connected formatted to FAT32 or HFS+?
It's 5.6.0 and I'm using a FAT formatted USB. I've been using the same USB for a while now, and the Sync Manager problem only happened recently
OKAY I FIGURED IT OUT. Turns out, one of the displays wasn't showing up. You have to drag the left sidebar open and then the sync manager pops up. In short, horrible UI.

I just shrunk mine right over and it stayed visible the whole time:

Is your display resolution set to something unusual or small? Do you have any accessibility zoom setting enabled?