I make a lot of markers and I get the "The file is busy and cannot be updated" error sometimes. But it happens almost every time when I make markers on a merged clip. I have an NVMe drive so I don't know how the drive could be too slow to do a file update, it feels like a program bug to me. I do use Dropbox and have tried turning dropbox off to see if that fixes it, but it does not. The only fix is to export the merged file, which is fine, but takes up more space and time. Sometimes, I'll make a ton of notes and then afterwards realize they have no text. It only happens with Merged clips though.
AMD Ryzen 9 3900X
32GB RAM
GeForce 1070 8GB VRAM
Windows 10
Premiere 15.2
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