Periodically users may run into an issue wherein files on the system cannot be copied or moved in the Finder. Upon dragging and dropping them with the mouse the file may start copying partially (if at all), but then stops with a warning, stating "Error -36".
This error message is perhaps one of the most generic in OS X, and in Apple's errors directory it writes that this means "I/O error (bummers)". Usually there is a small explanation stating a certain file cannot be read or written.
Essentially this amounts to the system having lost some sort of connection with the source of the file during the file transfer, which can happen because of errors with how the file (or aspects of the file) are handled during the move process, or because of actual I/O errors in the hardware.
Here are some approaches to solving this problem that may work if you encounter it:
Check permissions on the file
Go to the file in the Finder and check its permissions by getting information on it and looking in the "Permissions and Sharing" section for your username, the file's group, and "everyone." Make sure your account has read access to the file. You can try to take ownership of the file, or simply change the permissions for the group or "everyone" so they are at least "read," and then try copying the file again.
Remove extended attributes
First try removing extended attributes on the file being copied. These are beneficial to OS X, but may conflict with other operating systems, filesystems, and transfer protocols or technologies. To strip the extended attributes from the file being copied, open a Terminal window and type "xattr -d" followed by a single space. Then drag the target file to the Terminal window and when the path completes just press enter.
Re-mount external volumes
This problem may happen if there is an error in the external media or if the connection is not stable. If you have a USB or Firewire drive, try ejecting it and using another port if one is available. Additionally, if you have the option to supply the drive with external power then do that. You might also try booting into Safe Mode and then connecting the external drive to copy the files.
For networked volumes, try alternate methods of connecting to your network, such as using Ethernet versus Wi-Fi for the connection.
Move items using a means other than the Finder
This problem does seem to be spurred by the Finder, since file copying in the Finder is not as basic as moving the data from one location on disk to another. There are a number of checks being done, and interactions with permissions and extended attributes being performed that ensure the file copying is being done correctly. Though these checks are good to have, an error can prevent an otherwise intact file from being transferred.
If the Finder is not able to complete the file copy there are other methods that should work. One of these is using the Terminal, which is a pretty straightforward process.
First, open a new Terminal window and type "mv" followed by a single space. Then drag the source file to the terminal window. Then locate your destination folder in the Finder and drag it to the window as well. Once this is done, press enter and the file should be copied. Keep in mind that you will not see a progress bar, but the copy process will be done when the terminal returns to the command prompt.
For large files where the terminal will wait a while before returning to the prompt, you can check the progress of the copy by opening Activity Monitor and checking the transfer rate in the "Disk Activity" section. For a 1GB file being copied at 30MB/sec you would expect it to take about half a minute to copy.
Generally, once one of these steps proves successful for the file copy, the error will not crop up again. If it does then it is likely that a hardware problem is to blame, and you might try checking your hard drives for errors, or repartitioning them and formatting them (back up first!) to clear filesystem errors.