A family of Microsoft word processing software products for creating web, email, and print documents.
Opening documents from any kind of removable media risks document corruption. When you are working on a document, many temporary files are created. Not only do these threaten to fill the removable card or stick (though that risk is lower now that the capacity of USB sticks and the like is so much greater), but there is also a risk that they may not be deleted properly when you close the file and remove the drive (though it helps to use the "Safely remove" feature).
Although it may be irrelevant to this issue, since you are using a Mac, I will move your question to the Mac forum.