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.
Along with all the other suggested options, you may want to look at more basic hardware factors:

* try using a different usb cable
* try a different usb port on the computer
* try using the hard drive in a different external drive case
* if available, try using esata
* try connecting the hard drive directly to your motherboard and see if the problem occurs

And as always, if you think the drive is flaky, make a backup of the data on it asap. It will protect your data from a drive that may be dying and give you the option of doing destructive data tests if need be.