Error -50 is a very unhelpful generic communications error with a number of possible causes. Something between the drive to which you are downloading and the drive from which it is all coming is not working correctly, which probably leaves several hundred options. Here are some just to illustrate the range, though none seem to apply in your case:
http://discussions.apple.com/message.jspa?messageID=12744967 - user had two identical drives
HD downloads are Broken with a #50 error - https://discussions.apple.com/thread/3663076 - ISP had inadequate equipment for large download.
Error -50 and 8008 - https://discussions.apple.com/thread/3669623 - user trying to download iTunes video from a different country.
Getting error -50 when copying data in OS X. - http://www.lacie.com/support/faq/faq.htm?faqid=10127 - Problems when copying to FAT formatted drives.
You may have to copy the apps in smaller batches. You might have a corrupt file and you will need to isolate it to a specific file. I guess you could have a read permissions error too.