Error code -36 - cp: Input/output error - mac
In a Terminal:

grep "I/O error" /var/log/system.log


This will output to the Terminal all lines in the system.log file that has I/O error in it. Output such as kernel[0]: disk0s2: I/O errors are indicative of a falling and or damaged HDD. Note disk0s2 is an example and depending on which disk and partition has bad sectors the disk number and slice number will be different then the example. If you have output such as this and aren't backed up then do it before doing anything else!

Note; Depending on when the last time you had an error copying something it may not be in the current system.log file and you'll ether need to check the archived system logs or grep directly after the copy error.

If the copy is failing just on a particular file, you can attempt a recovery using ddrescue on the target file to try and copy it. ddrescue is a third-party utility that will need to be installed however dd is a part of OS X and it too can be used, although ddrescue offers more control and options then dd does.