I agree with Kevin that for really important things, you should have them backed up on optical media as well as another harddrive and possibly even another source. One of the most reliable back-up medias is still tape, believe it or not, and though expensive, they are invaluable for very important data. That said, with good compression, you could make multiple copies of things using inexpensive DVD-Rs.
As for your question, if C went down, F would still be perfectly readable since it is physically a different drive. If the two "drives" were nothing more than partitions (ie, divisions) of the same physical drive, things could get hairy. But if you're sure there is physically two drives in there, you'd be fine. However, there's nothing stopping F from going down one of these days and leaving C up and running as well.
For the record, I purchased a Seagate to replace the WD in that Acomdata case and much to my delight and surprise, Seagate drives carry a 5 year warranty. I'm buying Seagate from here on out.