I have to agree with those reporting previously. I have about 8 computers most with two internal HD’s each. In addition I have at least 12 external HD’s (USB and firewire) ranging from 100 GB to 1Terabyte. Some I put into external cases and some I purchased as external drives. I never purchase an external case unless it has a fan. So far only one drive has ever failed (a 80 GB external portable which was replaced one day prior to the warranty ending and the replacement is still running about 3 years later). I have had two internal drives fail in the past 20 years.
Key factors include excessive heat, running the drive when it is not needed, rough handling, lack of adequate power protection (I use all mine on battery back-up units), brand of drive, and bad or low power USB ports. All my drives are connected via powered USB hubs (8 HD’s and DVD burners to one computer) to insure adequate power even though the drive does supply power. I also have them connected in a chain through three USB hubs so they can be further away from my desk. People often forget to CLEAN their external drives - I blow compressed air through the fans to be sure there is proper air movement and open the enclosures when I can to blow out any accumulated dust. Also check the fans since they are small and can fail with no warning. I have never had a bad enclosure although a couple I have (with DVD burners installed) will not allow the firmware to be updated within the case.
The brands I have are Maxtor, Simpletech, Edge, and Fantom. The drives I put in cases are mostly Segate. The external cases are a variety of makes. I also have several older notebook drives in aluminum external cases (mostly 120 GIG) that operate with no problems.
I have (and recommend) a USB to IDE adapter that functions similar to an external case but there is no case. I use this to store files on a variety of internal type hard drives (much like flipping floppy disks or DVD’s) that I pile up on a shelf over my desk. Great cost reduction when drives are not needed all that often - just be sure to have good air flow and not use the drive too long. Also great for testing and preparing other internal drives.
All my hard drive image backups are contained on one 500 and one 200 GIG external Maxtor drives (need to get another terabyte drive for this soon.) I found DVD’s were to unreliable and too slow for this purpose!
So, I have been using external devices for many years (hard drive, DVD burners, CD burners) and find them to be very reliable if the proper care is taken. My portable ones travel to and from work every day in my briefcase - no problems. A couple of the external drives are connected all the time due to the purpose I use them for.
My philosophy is to have only a 200 gig or smaller internal drive in the computer and use larger external drives for the necessary storage. Having everything on an internal huge hard drive is just asking for trouble unless you do backups on a very regular basis! External drives are not running as often and thus it seems may last longer. On critical drives (external and internal) I run Spinrite on a yearly basis (or less) to check for surface defects and to refresh the magnetic tracks that store the data. The full surface scan can take hours but is worth it - use one older notebook computer just to do that.
Hope this helps. All hard drives will fail - it is not a matter of IF but of WHEN. Having backups is the best protection along with simple care. I have found that most times a hard drive failure is caused by rough user handling but they all must fail in time.