Most likely you are accessing the console using IE on one of Vista/Win7/Win2008/R2 and you have User-Access Control (UAC) enabled.
Temporarily disabling UAC while importing the definition fixes one of the causes of this error.
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Thanks for answering Chris. I'll try your solution and will feed back the results.
I had the same problem importing a list of device serial numbers which I solved by ensuring the first row of the csv is alpha-character only (a column name such as : 'value'). But ePO will import that 'row' along with the other values, so make sure you delete it afterwards. Any numerics in the first row will result in the error message: The file was empty or has data in an incorrect format.
Correct csv content example: