Losing USB functionality...

Software:
Windows Small Business Server 2003 SP1 with 10 users on a domain (used for file and print services)

Hardware:
Intel SE7525GP2 motherboard, dual 2.8GHz Xeon processors, 2GB PC 2700 DDRSDRAM, 120GB SATA RAID 1 set

Issue:
The server has been running good since June with no problems and this issue just started last week. There have been no major system changes other than Automatic Updates that I know of. There are two shared USB plotters (DesignJet 500 & 600) and one USB hard drive (WD 120GB combo drive) used for backups connected to the server. After the server is running for a while these three devices seem to "stop" working. There dosen't seem to be a set amount of time before they stop working nor does there seem to be anything specific that would interfere with the devices. The USB hard drive is connected to the USB in the front along with a keyboard and the two plotters are connected to the USB in the rear. The USB hard drive is powered down and removed every night after the backup and plugged back in at the start of every morning. The plotters are turned off every night but are never removed from the system.

When this issue occurs the USB backup drive can not be recognized by Windows (using My Computer, Disk Management and Device Manger) and the plotters fail to print immediately when sending a job to them, even a test page. If the server is rebooted everything works fine. That is until this issue happens again.

I am leaning towards it being a software issue because I don't think it is possible that both the motherboard USB ports have gone bad and the chasis mounted USB ports have gone bad. Plus there is a USB keyboard that is connected to a front USB port that works fine and that is where the USB backup drive is connected. I don't see anything related in the Event Viewer, no major software has changed on the server, and checking the history of Automatic Updates online says that none have been installed since August 10th. This issue started happening around September 5th.

I am lost as to where to go on this and any help would be greatly appreciated. Thanks in advance.