This is odd.
I have a Dell PowerEdge 600SC Server with 2.4GHz P4 and 4GB of ECC RAM.
The three internal drives work just fine: system is an 80GB ATA drive, and I have two matched 160GB SATA drives.
I added a USB drive and it was recognized immediately. I had no problems adding it to the Server Storage pool.
But when I plugged in a second USB drive (identical Seagate model, identical enclosure), it wasn't recognized. No amount of plugging and unplugging would cause it to appear in the console.
I went to the machine itself and saw that the drive was being recognized as an independent volume (drive E:) with all its files intact. I used Disk Management to completely delete the partition, but that did no good.
Finally, I decided to remove the original USB drive from the system using the console. That did the trick!