[karen cummings] “Does this mean that such a drive does not exist? If I know what my end editor will be using, then I can format for that but is there no drive that can live in both systems? “
Unfortunately there is no magic drive format that works equally well on both platforms – it’s a compromise. NTFS is a Windows file system, and HFS+ is a Mac file system. Macs can natively read, but not write to NTFS formatted drives without additional software. Windows PCs cannot natively read HFS+ formatted drives, but can do so using MacDrive software.
There are NAS (network-attached storage) devices that use other file systems (like xfs) that are platform agnostic, but NAS devices are not recommended for editing because their transfer speed is not fast enough for adequate editing performance.
Your only real options are to use an NTFS formatted drive (with the appropriate software on your Mac) or an HFS+ drive (with MacDrive on the PC). Either option would work, but if you don’t want your client to have to purchase the MacDrive software, then NTFS is probably the best option.