My backup solution for home & office including my server is 2BrightSparks' SyncBackSE, about a $30 program that has proven itself invaluable. Since your scenario probably most closely resembles my office, here's the plan. I chose (2) 2.5" laptop drives (with the ability for an external power brick) about 2-3X the amount of data I was backing up, in this case 100GB for less than $100. Then I bought (2) Thermaltake cases and (1) power adapter. Every day, at 11 pm, ntbackup backs up system state and My documents, at 1:30 SyncBack backs up all other files and in your case, Quicken.
At the end of day, the accountant goes to the server, double-clicks that little "Safely remove Hardware" icon, and removes the drive. If it says the drive cannot be stopped, logging off and back on will fix that. I then have her switch drives and take the other drive with her, so there is always offsite media.
For my home, I have a ginormous external USB drive, also wall-powered, that stays off most of the time. About every week I back up "Documents and Settings" for the (3) PC's in my home office. It's an auto-run routine that simply doesn't run when the drive's off. SyncBack is an incremental file-copy backup so it can restore and accidentally-overwritten file. In the event of a disaster all I need to do is take the drive.
The two most important points I would like to stress are not relying on USB-power and stopping the device before removing it or tuning it off.
Tony