Answer Best answer as chosen by user dennis123138
What's the betting that your external hard drive was
previously used with a Windows machine?
If the answer is "yes it was", then there is a distinct possibility that it is formatted as NTFS, standard for Windows, and as such is readable by the Mac but not writable.
Which means that you can see what is on the external drive and copy it to your Mac but you cannot go the other way.
To get around this, you have a couple of choices.
1. Reformat the external as Mac OS X Extended (Journaled) Standard for Mac
2. Download a copy of NTFS for Mac (Not Free)
3. Download a copy of NTFD-3G for Mac (Free)
Using any of those choices will enable your Mac to Read and Write to a disk that is formatted as NTFS
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