There are three good ways to share a single hard drive between Macs and PCs for general storage and backup: You can trade a USB hard drive between them; you can use a network-attached storage (NAS) device (such as Apple's Time Capsule); and you can also use file sharing to back up data on one system to an external drive attached to another system.
Swap a USB Hard Drive
Macs and PCs both have USB ports to which you can connect a removable hard drive. The only sticky issue is formatting.
If the drive is formatted for the Mac (using HFS Extended or a variant), Windows won't recognize it. You can fix that by installing Mediafour's $50 MacDrive 8 on the Windows system; it enables Windows to use a Mac-formatted drive natively.
If the hard drive is formatted in the Windows NTSF format, it will mount on the desktop when you plug it into the Mac, but you won't be able to copy files to it. OS X can read NTFS drives, but it can't write to them. You can fix this by installing Paragon Software's $32 NTSF for Mac OS X. If you'd prefer to save money--and if you're adventurous--you can try two free programs: NTFS-3G for Mac OS X and MacFuse, which must be installed together.
There is a third format that both Mac OS X and Windows can read and write to natively: FAT32. It's also the format that most USB flash drives use. FAT32 has some limitations: It can't store files bigger than 4GB, you can't boot a Mac from it, and it's slow. If you want to use FAT32 on a shared USB drive, use Mac OS X's Disk Utility to erase and reformat a hard drive in what Disk Utility calls MS-DOS (FAT).
Connect to NAS
Macs and Windows PCs can use almost any networked-attached storage drive over a wired or wireless network; Apple's Time Capsule is actually one of the best choices for shared backup.
Time Capsule is formatted as a FAT32 drive, so Macs and Windows PCs can both read and write to it. For file transfers, it supports the SMB protocol (used by Windows and Linux systems) as well as the Apple Filing Protocol (AFP).
The Time Capsule installer CD-ROM includes Windows versions of AirPort Utility and the Bonjour Printer Wizard, so the PC can set up and manage the Time Capsule and any printer attached to it. No Windows version of Time Machine exists; any Windows backup program should do.
Most non-Apple NAS devices are also formatted as FAT32, but you should check before you buy. You may be able to reformat an NTFS NAS device later. Non-Apple NAS devices usually support SMB file sharing only. If you can't see such a device in the Finder, try typing its network address--smb://ipaddress--in the Connect To Server dialog box.
Share an External Drive
The third way to share storage between Macs and PCs is to plug a USB or FireWire drive into the Mac, and then have the PC back up to it over the network using file sharing.
The only difference between this and the file sharing described earlier is that you must specify the external drive on the Mac while the drive is connected.
To do so, open the Sharing pane in System Preferences. Select File Sharing in the left column. Click Options, make sure SMB is selected, then click Done. Back in the Sharing pane, click the plus-sign (+) button under the Shared Folders column. In the left panel under Devices, select your Mac, then select the external USB drive. Click the plus-sign button. You can now set the read/write privileges for the shared drive as you would for any folder.
John Rizzo is the publisher of MacWindows.com and the author of Snow Leopard Server for Dummies (Wiley, 2009).
This story, "Living With Windows: Sharing Drives" was originally published by Macworld.