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The only reason this is important is the actual physical connectors; you cannot use one in place of the other (you can, possibly, but not in a laptop because there isn't the room to add all the extra bits). This stuff is not like understanding your basic home broadband connection or the installation of a dvd drive, so don't attempt to understand what IDE and SATA means. You will end up in a black hole along with the RS232 mystery, just leave it, it's not worth it.
Despite not needing to know, or even be interested in knowing, what IDE and SATA stands for you do need to be able to tell the difference.
This picture shows the contents of a typical external 2.5" unit. Note the yellow boxed area, this is where the hard drive connects to it's interface pcb (printed circuit board). I have pulled the two apart and drawn a bluish line where the two push together, to make it a bit clearer than it might otherwise be if I had photgraphed them mated. This is an IDE connection.
If it's IDE then there is just one single block connector with 40 pins. If it is SATA then there is a similar connector but the two are not similar enough to be interchangable. The drive itself is identical or almost identical but this connection, IDE or SATA, makes a crucial difference that must be known before you buy.
A visit to a couple of well known UK shops that sell a range of computer equipment confirmed for me that it is not possible with any of the six or more 'makes' of external, housed, "portable" (read 2.5") drives, to discover by looking at the packaging what kind of drive (IDE or SATA) is inside the housing.
This means that you would have to make your own enquiries - you could ask a sales person but the trouble with this is that you are then going on trust and it may be a year of more later than you attempt to swap the drive in which case you are stuck and your warranty will possibly (some are more than a year with these items however) have ended.
By Paul E. Coughlin
SaneThinking.com
27 May 2008