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it will have to be formated to work with Mac OS, otherwise you won't be able access it or depending on the format, you may be able to read from it, but not write to it.

as long as it's a new disk, this isn't a problem. Formatting is easy, but will erase existing data.
if it's been used before, you may run into some issues if you want to save the data on it, but don't have enough space on other drives to back everything up before you format.

here's a good article explaining the different types, it's talking about a thumb drive, but the formats are the same.
 
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