The latest version of the continuous backup and syncing software NTI Shadow, available for both PC and Mac, adds syncing in both directions, encryption, and other new features. And it's on sale through the end of this week.NTI Shadow is in the category of backup software that works by syncing a folder or multiple folders on a source hard drive with a specified destination drive or folder. The backup/syncing process is continuous, as long as the destination drive is mounted on the source computer. One advantage to this approach is that files are mirrored in their native format on the backup drive, so restoring files just means copying them back to the source computer. Another is that it provides an easy way to keep files on two computers in sync, for those businesses that prefer not to rely on a cloud syncing solutin such as Dropbox or SugarSync. To sync folders on a work and home computer, for example, just requires syncing from one to a portable hard disk and then syncing from the disk to the second computer.

Version 4 of the software adds two-way syncing that supports multiple drives and multiple computers, making the process of keeping several computers synchronized that much easier and more flexible. The destination and source folders can be kept in perfect sync -- so that files deleted in the source are also deleted on the destination -- or the destination folder can continue to accumulate new files while preserving the old. Shadow also implements versioning to make it possible to locate previous versions of backup files.

For Windows users, Shadow can create an encrypted partition on an external drive, such as a flash drive or portable hard disk, and sync to that, adding an extra level of security for files carried from one place to another. Mac users can choose to sync to an encrypted .dmg image instead.

The fact that Shadow runs on both PCs and Macs opens up new possibilities for cross-platform syncing. A folder on a Mac, for example, can be synced to a portable drive or a network-attached drive, and then a PC can be synced from that drive. In a networked environment, users can even sync from a Mac to a mounted, shared volume on a networked PC, and vice versa. (Each computer needs its own copy of NTI Shadow.) Other syncing software is available for the Mac, such as ChronoSync, FileSync, SuperDuper, and Carbon Copy Cloner -- and no doubt similar software is available for the PC -- but NTI Shadow's cross-platform capabilities make it a standout option for multiplatform environments.

NTI Shadow 4 for Windows works with Windows 7, Vista, and XP; and the version for the Mac works with OS X 10.4 (Tiger), 10.5 (Leopard), and 10.6 (Snow Leopard). Its regular price is $39.99 with a 30-day free trial, but it's available through May 23 for $19.99. The software can be downloaded here.

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