using USB2 external hardrives for LAN backup

I am using several USB2 external hard drives (200 GB to 200 GB)
for backing up my network to. The full backup is about 150 GB. I
use robocopy to copy all of the drives to my external drives.

I have reformatted all of the external hard drives to NTFS. I am
running Windows XP SP2. I am using an ASUS A8N Premium
motherboard with an AMD 4800+ dual core cpu and 2 GB of ram.

Usually after about 30 to 50 GB is copied to the external hard drive,
my computer starts getting this error message when robocopy
tries to copy, "insignificant system resources ...".

Does anyone know why I am getting this error message ? Is USB2
robust enough to have 150 GB written over it during a 24 hour period ?

I am also getting this problem on my file server running Windows
XP SP2 with dual AMD Opteron cpus and a Tyan motherboard.

Thanks,
Lynn