Can run afoul of cross-device link error

gistfile1.m

NSURL *remoteFileURL = [[self temporaryDocumentsURL] URLByAppendingPathComponent:ZTDropboxRemoteFileName];
NSURL *databaseFileURL = [[self dropboxFolderURL] URLByAppendingPathComponent:self.dbFileName];
NSError *error;
BOOL success = NO;
NSFileManager *fm = [NSFileManager defaultManager];
// if this isn't our first time, use replaceItem...
if ([databaseFileURL checkResourceIsReachableAndReturnError:&error])
{
   NSURL *newItemURL;
   success = [fm replaceItemAtURL:databaseFileURL
                    withItemAtURL:remoteFileURL
                   backupItemName:nil
                          options:0
                 resultingItemURL:&newItemURL
                            error:&error];
}
else // just do a plain-ol' copy
{
   success = [fm copyItemAtURL: remoteFileURL
                              toURL: databaseFileURL
                              error: &error];
   // ...
}

self.dropboxFolderURL is chosen by the user with NSOpenPanel running in Sandbox, so it is security-scoped. It's either supplied directly by the user, or retrieved from a security-scoped bookmark stored as NSData in NSUserDefaults.

When replacing an item on an attached volume, you get:

NSUnderlyingError=0x105b0abd0 “The operation couldn’t be completed. Cross-device link”