Learning from the accident
DO NOT trust Windows explorer's copy function. Use FastCopy and verify your file.
Pause the sync when you keep changing your files, no matter what sync client you use: Google Drive, MountainDuck, or dropbox.
Use something like 7z and rar to provide some abilities to check file integration.
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Today I got 30+ files with 0 bytes (the file should be 5MB~10GB), and about five files with missing content (should be 20+MB, but only got first 5~6MB synced).
Those are files I copied from Google Drive and pasted to MountainDuck. I don't know how I got those broken files, but I think it's related to Google Drive. I had a similar experience where the file on Google Drive is good, but downloading gives a broken file. And the sync part, the MountainDuck seems like using timestamp only and choosing the newer file, even if the file in the local disk is bigger while the last modified timestamps are the same.
Learning from the accident
@skyblond I've already put my NextCloud on pause on my notes from Obsidian because NC kept complaining about locked files since Obsidian writes way too often to those .md files. But other than that, I've not really had issues with Windows' built in copy. I do, however, always make sure to disconnect the USB devices via software first before I take them out.
Learning from the accident
Using FastCopy, even if Google Drive returned an unexpected EOF, the verify stage would find it and let me know (unless two unexpected TCP RST occurred at the same position and both failed the retry).
With Windows's copy, I just assume all files are properly transferred and delete the original file. With 7z or rar, I can verify them. But with other normal files, like mp3 or flac, I can only notice that when the music stopped too early.