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).
If you do find out that a HDD is failing, you may want to check out [Recovery Is Possible](http://freshmeat.net/projects/recoveryispossible/).