OS X V10.6 Snow Leopard :: Copying Files Between Hard Disks Interrupted By Error Code -36

This error probably indicates a hardware problem or some system malfunction. I usually try to find out what exactly went wrong to take measures to avoid the repetition of the same problem. Exchanging the HD and restoring the sytem may be the quicker solution, but I would miss an opportunity to reconfigure my system to make it more robust. 

The system in question is a MacMini (Model A1347) with Os-X-Server 10.6.7. and 2 physical HDs: HD1 has 2 Partitions, the smaller for the server OS, the larger for the Shares (We use the MacMini only as a file server). HD2 is not used (was kept in reserve for cases like this).  

That there may be a problem manifested itself first when the mounting of the shares on HD1 became suddenly slow (within 2-3 days) and eventually failed. I first tried to copy all the files from the shares on HD1 to HD2 before attempting any repair actions. Most of the files copied properly but a few failed with error (-36). Those few can be restored from backups, so there is no data loss. Remains the question, what went wrong?  

It may be pure coincidence but just before the File-Server failed there was an attempt to upgrade a 3rd party software on one of the clients (17'' MacBook Pro / OS X 10.6.8) to a newer version. Could be that new version was part of the problem because the data files of this 3rd party software are stored on one of the shares on HD1 and the data file format has been changed between the versions. For obvious reasons I need to make sure that this data file conversion was not the cause of the problem?

1) first check the HD1 with a different tool than DiskUtility (It thinks the HD1 is in perfect shape), TechTool Pro or DiskWarrier comes to my mind, any reason to prefer one over the other? 

2) Hopefully those tools can reliably test whether the HD1 is damaged or not. If it is the former the HD1 replacement option is the way to go and there is no more reason the 3rd party software may have been involved. However if HD1 is not damaged I'm left with the uncertainty of what exactly lead to the problem..? 

3) Is there any check I should do before I just re-format the HD and re-install the OS and switch back to  an earlier version of the 3rd party software to be on the safe side?  

Info:
Mac OS X (10.6.7)