Depending upon the application you're using to copy across, it may be that you have a default set to convert to JPG.
How is Photoshop involved in that operation?
…or whatever method or software you're using to "copy" the files to your external HD is somehow saving "copying" an embedded JPEG thumbnail or a freshly generated one.
Where do your raw files originate? How did you get them onto your MacBook laptop? What exact version of Mac OS X are you running?
If Photoshop is in any way involved, please indicate the exact version number of that application.
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