Bought a 3TB Seagate usb portable harddrive, and wanted to partition it for both Macs (to have Time Machines on their own dedicated partition) and Windows (for general storage of files).
Can’t be done using Windows, at least not the free software I tried. Had to be done on Mac, but not without some strange phenomenon that required a few tries. Basically, Mac’s Disk Utility won’t partition the Seagate as-is, from what I can find its most likely due to a EFI that is smaller than 200MB (in fact, the Seagate doesn’t have a EFI partition), and also the Seagate has a Microsoft reserved partition that seems to serve no purpose.
Whatever the reason, I didn’t bother to figure it out. I decided to forgo Seagate’s default setup and start from a blank disk. So, in the Mac’s Terminal:
diskutil unmountDisk force disk2 //or whatever disk# the usb drive is]
sudo dd if=/dev/zero of=/dev/rdisk2 bs=1024 count=1024 //this will erase the entire usb disk
diskutil partitionDisk disk2 GPT JHFS+ "KnockDrive" 0g // this will partition the drive with a EFI and the remaining space as a partition of Apple's HFS format
Now open up Mac’s Disk Utility. Use the Partition function to segment the desired number of partitions; for the partition that will be used by a Mac, set it as Journal HFS, and for the partition that will be used by Windows, set it as exFAT.