Upgrade the Linksys firmware to something that properly supports USB mass storage class devices, it seems their current implementations lacks full and proper support, much like Windows XP and prior; it will only support up to 2TB of addressable space.
Abstract from WRT610N User Manual p20 :
Select the disk you want to format, then click Format Disk to format the disk and create a new partition. If your hard drive is formatted with multiple partitions, it will delete them and create a single partition. Enter the name of the partition you want to format. Select Format to start the formatting or Cancel to cancel it before it begins.”
Seems to imply it supports only a single partition.
Too bad, the comparison with Apple Extreme Base Station would have been interesting.