Unless the USB device is in a locked closet/computer room, it may be more ""portable"" than you'd like (along with clients data). It's part of the solution, but not the solution.
I wasn't trying to back up the entire network, but quickly realized that it was more than just the mas90 folder. Other issues were related to backup software itself. Some days I'd get confirmation e-mail stating it failed. Now you have to troubleshoot why, then try the backup the following night. Perhaps another error or one that asks you to reinstall the entire backup software. Perhaps no confirmation e-mail is sent? Most apps push updates just like GotoAssist, so between them and Windows updates, you soon find yourself spending many hours troubleshooting. Many of these issues can't be resolved during work hours either. Multiply this by half dozen clients, and you've got a part-time job (unpaid or at reduced rates... cause IT can do it for $120 or less). Then the ""network throttling"" feature the software promised doesn't work quite right, which earns you a call because your backup software just brought the network to it's knees. Or client creates 15 test companies and quickly runs out of storage... which means you have to define inclusions/exclusions for what you back up, then teach the client cause each saved history company (at year-end, quarterly payrolls or during month-end closing) will have to be manually added to the backup set.
Other than that, it's pretty solid.