iTwin doesn't rely on a remote server farm to store your files and a password-protected web interface or software package to access them.
It consists of a pair of identical USB dongles; plug each one into a different internet-connected computer and install the included drivers. Each computer then has a pair of virtual disks accessible from within Windows Explorer: Local Files and Remote Files. Any files copied to the Local Files disk on one computer can be accessed from the other computer through its Remote Files disk, and vice versa. This connection works as long as each computer has its iTwin dongle connected.
Although iTwin requires that both computers are connected to the internet, files are encrypted and transferred directly between the two. Files are stored on iTwin's relay servers only temporarily in case of adverse network conditions. Since iTwin relies on your computers to store files, there's no storage limit. The only limit to the number of files you can share is the size of the hard disk on each PC.
If an iTwin dongle is lost, disconnect the other dongle to prevent unauthorised access to your files. If you want to guard against unauthorised data access, you can set a password on the iTwin; this means files can be accessed only if you have both the dongle and password. The password can also be used to disable a dongle remotely. iTwin won't suit everyone. It doesn't work with Macs, Linux or any smartphone/tablet operating systems. It also won't work if you want to share files between more than two computers at once. Still, if you're concerned about privacy and you don't want another set of passwords to remember, iTwin is a good alternative to more conventional online storage and remote access services.