First see how the hard drive is formatted
Right Click on the Hard Drive and Choose
Hard Drive should be Mac OS Extended
Click on the Lock to change permissions
Make sure Ignore Ownership is checked
How To Format a Hard Drive for Macintosh Computers
Open Disk Utility
Go to Macintosh HD
Pick the drive you want to reformat
Choose the Volume Format: as Mac OS Extended (Journaled)
Name the drive then click
Erase in the lower right corner to erase the drive
How to format a hard drive in Windows.
Plug in the external drive to the USB port and turn it on.
If the following wizard opens
Click on cancel. If the disk says Not Initialized then right click on the drive and initialize it by clicking on Initialize Disk. Click OK at the next menu to complete the process.
The next step is to format. Right click the drive in the stripped area and click New Partition
Click Next at the Convert Disk Wizard
7. Select the Primary partition and click Next
8. Specify Partition Size to the default which is the capacity of the drive and click Next
9. Assign a drive letter or choose the default then click Next