The problem is, if your HD was formatted as NTFS, OS X will be unable to write to the drive, since only "Read" ability is enabled for NTFS under OS X as the default. It is possible to write to NTFS-formatted HDs IF one uses a third-party utility such as NTFS-Mac 9.5 [by Paragon Software] or one of the free utilities which enable NTFS writing ability under OS X.
IF you do not wish to purchase a third-party utility, or use one of the free utilities, the only course of action you have available is to copy your files to your Mac drive, prepare the external drive as "GUID", then move the files back to the external drive after it is enabled as a Mac volume. To do this, select the files you wish to save, then just drag them to your Mac drive. I suggest you create a new folder on the Desktop and drag them there. After all is done, you can just drag the temporary folder to the Trash Can.
1) Open Disk Utility [found in ~/Apps/Utilities].
2) Make sure the external drive is mounted. If it's greyed-out, click on "Mount" in the Menubar at the top of the screen, and it should change to darker text. [If It's is still greyed out, stop at this point and we can attempt to try to find out why it won't mount].
3) Click on the drive designation itself [NOT any partitions on the drive]. The drive designation is the parent of any partitions under it. Make sure the "Partition" tab is selected. At this point, there will be a window displaying the partition scheme on the drive. If you do not make changes to the partitioning scheme at this point, it will not change to a Mac-writable one.
4) Click on the arrows beside "Current", and a drop-down list will allow you to select the number of partitions you want. If you do not select any [leave it as "Current"], no changes will be made to the partitioning scheme, even if you click on "Apply" at this point. In your case, it will leave you right back where you started, with an NTFS volume and no ability to write to this volume from within OS X.
5) Below the partitioning scheme area, there will be a button labeled "Options" now enabled [it is greyed out until you change the partitioning scheme in the list]. Click this button, and a window will open giving you the option to erase the partitioning scheme and make it "GUID" -- the first radio button. If you do not name the new partition, it will be given the default scheme of "OS X - Journaled", with the name "Untitled". Make sure this radio button is enabled. If you don't want any of the volumes you create to be bootable, activate the second radio button, and any partitions you create will only be for storage, but not readable/writeable under Windows. If you want them to be read/writeable from within both Windows and OS X, select the bottom radio button ["Master Boot Record"], since FAT32 is pretty-much a universal partitioning scheme.
At this point, Click on "OK", then "Apply", and the partitioning scheme will be created that you selected in [Step 6]. Disk Utility will then work for a minute or two, and when it is finished, you can copy or move the files you saved earlier back to the partition you created.
Donald L McDaniel
OldCelticHippy
I've recently switched from a PC to Mac and I had a friend put all my music and pics onto a "Mac compatible" external hard drive ( G-Drive) but after plugging it into my Mac I've found that all the files are read only.

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