has not been formatted as OS X Extended and may well be formatted as NTFS.
The Mac can read (see) files on an NTFS but cannot write to it. To delete files, the mac has to write to the drive.
Suggestion.
Create a folder on your desktop and copy the contents of the Seagate into it.
Now use Disk Utility to format the drive as OS X Extended (journaled)
Once this has been done, you will find that you can now do everything you want to with the drive, including Time Machine.
Just put all the stuff that you copied off the Seagate, back onto it and you are back in business.
Let us know if that worked out for you
P
I have a macbook and have the Seagate Free Agent external storage drive. However, when I open free agent, I cannot delete a folder, nor can I add a folder from another source. What am I doing wrong? I need to keep copies of everything on my computer and need to know the best way of doing this. I thought the free agent would help. I have timeline on my laptop but it doesn't seem to work with any of the drives I ask it to download to. I am new with the Mac, so I am hoping I can get some help. Thanks.

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