Since I know that I’ll never plug this drive into a PC, I’ll choose a Mac disk format, and of those, extended journaled is your best bet. If you wanted to have it be PC compatible, use MS-DOS format, but be warned that my experience is that read/write speed suffers on the Mac because of it having to reorganize data as it hits the drive.
Give the drive a name — I chose “Insurance” for mine — and click on “Erase…”
Read more at http://www.askdavetaylor.com/external_mac_hard_drive_read-only_how_to_fix_it/#Xai5GsssPtD7MWum.99