Using windows explorer one cannot drag n drop (cut-paste) documents across site collections.
If you try to cut-paste (drag-drop) documents across site collections or across sites in same site collection, you would get the following error: -
"cannot read from source file or disk"
1) Open the source and destination libraries as network places. (using add a new network place).
2) Browse to the document libraries in question.
3) Drag-drop the files.
4) There’ll be no error.
Things to check at client’s end: -
1) Check if Webclient service is running and is set to auto start in services.msc. Stop and restart it.