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: 
The [Ultimate Boot CD](http://www.ultimatebootcd.com/) has some Hard Drive tools on it that should detect any errors on the drive. Just make sure that you back up all your data FIRST. Doing intense testing on a bad drive may lead to worse failure and data lost.
If you do find out that a HDD is failing, you may want to check out [Recovery Is Possible](http://freshmeat.net/projects/recoveryispossible/).