1. Reliability/Quality.  Why do you want an Ext HDD?  To backup your data!  If the reliability of the Ext HDD you purchase isn’t as good as the system hardware from which you’re backing up, you’re wasting your time (& money).  In my opinion, retail External HDDs — especially the more affordable ones — are becoming increasingly unreliable.  Not only are the cases cheap, but the controller electronics, external connectors, power supplies, etc., are all of inferior quality.  The Seagate Free Agent Ext HDDs I mentioned in my previous post would get extremely warm… and this was when I wasn’t copying any files.  Heat is the enemy of most all computer components, especially HDDs.  Most of the external HDD enclosures available for building your own Ext HDD are often all metal (which is much better at dissipating heat than plastics) and often include cooling fans.  Keeping your HDD cool will noticeably increase the odds that it will survive many years of use.  I have also found the controller electronics, connectors & power supply to be of superior quality in separately purchased external enclosures.  If you read the customer reviews of many of popular retail External HDD products (i.e., Newegg), you’ll see a significant level of both failure and customer dissatisfaction.
  2. Data Recovery.  Should the HDD in your external enclosure fail, you have a better chance of recovering the data than with a retail Ext HDD, unless you’re willing to crack open the case of your retail Ext HDD & remove its HDD (which in almost all cases, voids your warranty).  You have fewer options for file/data recovery via USB than if you remove the HDD and connect it directly to your system (via motherboard IDE or SATA connectors).  Usually your only option is to download the Ext HDD manufacturer’s “disk tools” software, which in my experience “may” be able to restore the HDD to use, but is terrible at actually “recovering” any files.
  3. FlexibilitySay you decide to upgrade the HDD(s) in your desktop PC, i.e., removing your two 320GB HDDs and replacing them with two 1TB HDDs.  What do you do with these 320GB HDDs after they’re removed?  Well, if you have more space & connectors in your desktop PC, you could use them there.  But if you don’t, you can use these HDDs in another external enclosure for yet another Ext HDD, or if larger than your current Ext HDD so you could just replace it’s HDD with one of these.
  4. Disk Imaging Say you want to make a complete image of your system disk.  You first need a HDD of equal or greater size of the disk you want to image.  Next you need a means to connect the HDD to your system.  Again, if your desktop HDD bays are full or you don’t have any more motherboard HDD connectors, you can use your homemade Ext HDD.  Just remove the existing HDD from the Ext HDD Enclosure & install the HDD you want to place the image on, and go ahead and image your system disk with whatever software you choose (Acronis, Paragon, Ghost, DriveXML, etc.).  When through, you can remove the image HDD from the External HDD Enclosure & reinstall the previous HDD.

I cannot emphasize the importance of performing backups… that is, if you keep or have any data on your PC that feel you couldn’t live without or is important to you.  This might include photos, financial data, personal data, emails, music, videos, etc.  Murphy’s Law will ensure that the more important the data, the more likely a HDD will fail IF you don’t have at least one backup of that data.  I have not lost any data over the past 18 years, nor have I had a HDD fail on me (except for the three Seagate Ext HDDs mentioned in previous post).  In my experience, HDD failures almost ALWAYS occur — in both home & office environments — when you DON’T have current/multiple backups.  It’s like rain & umbrellas.  How many times have you brought an umbrella with you & not needed it?  How many times have you encountered rain when you DIDN’T have your umbrella?  For me, it’s about a 10:1 ratio (meaning I’ve encountered rain when I didn’t have my umbrella 10 times more often than when I had my umbrella with me).  Consider “backups” your safety umbrella.  You never seem to need them when you have them… but I can almost guarantee — if you don’t have a backup, you’ll eventually REALLY wish you had!