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Your external drive is formatted in the Windows NTFS format which OS X, the Apple Mac Operating system, can READ From but can NOT Write to.
You will need either another external drive or enough space on your internal Mac hard drive to Copy all the folders and files from your current external drive and then Format that external to the Mac OS X format, Mac OS Extended (Journaled), and then Copy and or move those folders and files back onto your current external drive.
If you use another external drive make sure it is formatted either in the Mac format or the Windows FAT32 format or the MS Ext32 format.
99% of all external drives sold today are pre-formatted in the Windows NTFS format. So if you use another external you must first format it to what I have listed above before you try to copy or move files to it from your new Mac.
NOTE I said Copy your folders and file from your present external. Don't try to Move them as to Move any file it is first copied to the new location then Deleted from the old, original, location. Since OS X can Write to NTFS formatted drive, which is needed to delete files the Move will fail.
Thank you so much!