I would run Apple Hardware Test: http://support.apple.com/kb/HT1509
If you have a backup - Time Machine or the equivalent (your Western Digital USB appears to be your backup disk) then I would not worry, not a bit.
Even if your internal drive were to fail catastrophically, having a backup will make recovery a trivial event.
Goes without saying though, make sure your backup disk is OK!
& thanks for setting my mind to ''peace & tranquility mode'' ;-)
one more question though : does that have anything to do with Temp sensors or so ?
i keep my iMac switched on 24/7 & never really noticed it getting HOT ( warm ? yes a bit but.. ) & NEVER actually heard it's cooling fans
have a good week-end
My iMacs have the i7 CPU like yours. I have actually tried to make them overheat by running Geekbench stress tests. The temperatures will rise to about 60 to 65°C and stabilize around there, and I never hear the fans run, even after hours of continuously running the stress tests. The iMac gets a little warmer but not much.
This concerned me at first, since I thought perhaps the temperature sensors or the fans were not working properly, so I ran Apple Hardware Test. In fact the AHT exercises the fans and they can get quite loud.
So I conclude the i7 CPU is very effectively cooled in these Macs and / or it is just a very efficient processor.