Are the external drives USB or Firewire? They're big drives, so I'm assuming that they are both externally powered and you're not trying to run them without external AC. If you can boot and then mount the drives, try running Repair Disk from Disk Utility to see if there is a problem with the disk(s). The slow mounting disks are an indication. Are you booting from one of these disks or from the internal drive? If you are booting from one of these disks, try repairing permissions from Disk Utility. Even if you are booting from the internal drive, repairing disk permissions on the internal drive won't hurt either.
Steve