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: 
I thought [SpinRite](http://www.grc.com/spinrite.htm) was free, but I guess it isn't any more. Based on that I would use the manufacturers diagnostics tool, see samsungs [here](http://www.samsung.com/global/business/hdd/support/utilities/Support_HUTIL.html).
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.