The error occurs because you are trying to move the contents of the D: drive back to C: as if the entire D: drive were your Downloads folder. The folder “D:\System Volume Information” is a protected Windows system folder. It is intentionally inaccessible to normal users, so Robocopy, File Explorer, or another copy/move operation that scans the entire D: drive can report “Access is denied.” You should not change the permissions on System Volume Information or try to move or delete that folder.
The likely issue is that the Downloads folder was redirected directly to the root of D:, rather than to something such as “D:\Downloads.” When that happens, Windows treats the root of D: as the Downloads folder. Consequently, when you try to move the Downloads contents back to C:, Windows or a copying utility can encounter other folders on D:, including protected system folders.
First, check where Windows currently thinks your Downloads folder is located. In File Explorer, right-click Downloads under your user profile, select Properties, and open the Location tab. If it shows something like “D:\” rather than “D:\Downloads,” that explains the problem. The safer configuration is to have Downloads located at “C:\Users<your-name>\Downloads” or, if you want it on D:, “D:\Downloads.”
If you want to put Downloads back on C:, use the Location tab and select “Restore Default.” It should change the location to “C:\Users<your-name>\Downloads.” Click Apply. Windows may ask whether you want to move the files from the old location to the new location. If it does, allow the move, but make sure you are moving only your Downloads files, not the entire D: drive.
If Windows refuses to move them because of the System Volume Information error, do not take ownership of that folder. Instead, create a temporary folder such as “C:\TempDownloads” and manually copy only your personal downloaded files and folders from D: into it. Do not copy folders such as “System Volume Information,” “$RECYCLE.BIN,” or other Windows/system-created folders. Once your personal files are safely copied, you can set Downloads back to its default location and then move the files into “C:\Users<your-name>\Downloads.”
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hth
Marcin