When multiple users share files, they may make changes simultaneously, creating conflicts. All of these services except for Windows Live Mesh 2011 will notice if a file is synced at the same time from two different parties and then create conflict copies (typically labeled by user or machine name). But none of them achieve the level of version-control that's offered by Google Docs, which tracks all changes behind the scenes and applies them live to all users at the same time. Even when glitches occur, putting files out of sync, Google warns you and asks you to close and re-open the file.
The one thing these services won't let you do is lock individual files--preventing anyone from editing a file that someone else is already working on or making it impossible for specified users to ever modify a given file. Those features are commonly found in content-management systems (CMSes) that are specifically designed to handle such documents. Shared storage and syncing services aren't focused on the contents of documents as much as on saving a current copy of a file each time it's modified. (Some services let you mark files or folders as read-only--for everyone or particular users--but that's not the same thing as locking a file.)
Permissions and security
These services vary widely in the granularity of their file permissions. For example, Dropbox lets you set permissions on an entire folder, not for individual files. Furthermore, Dropbox's permissions give all users editing rights; other services let you grant view-only access to specific items or folders; some offer even finer control than that.
Many of these services let you share a file or folder with anyone for whom you provide an e-mail address, even if he or she doesn't have an account with the service; in some cases, that outsider can also share the file or folder with others. Some of them also allow you send out public links (which enable anyone to download a file from the service's Website) or links that only the recipient may use or that works one time only.
Except for Windows Live Mesh, all of these services save older versions of files. However, they vary in how many versions they store and in how they count those older files against your storage quota. Most will track who has made changes to a file. Being able to retrieve older versions is particularly important when you're sharing files with a group; while you may have local backups of the files you've used, cloud-based tools save changes more frequently, making retrieval of previous versions easier.
In an age of server break-ins, encryption protocol exploits, and man-in-the-middle attacks, any online storage service aimed at business users should secure data in transit as well as in storage. Microsoft provides less information about its security measures than some other services.
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