Question: Are there any free tools available to perform hard drive diagnostic?
I have an old 3.5 IDE Samsung HDD which I use as an external drive through USB connection. I recently started to have problems with accessing some of the data.

Answer: 
Wikipedia has a list of [harddisk diagnostic software](http://en.wikipedia.org/wiki/Comparison_of_S.M.A.R.T._tools). These programs are called S.M.A.R.T. tools, they detect if some of the reliability parameters are not okay. I used the free [Active@ Hard Disk Monitor](http://www.ntfs.com/disk-monitor.htm) to run checks on my harddisks.
You can grab Hiren's boot cd which contains some free tools for this purpose. [Website is here](http://www.hiren.info/pages/bootcd)

Maybe you'll have to attach your drive to the IDE bus for doing your maintenance work.