Unfortunately i don't have any external hard drives, so i can't answer your question fully, but i have tried connecting two USB flash drives and have made some observations.
I normally use a 4-port non-powered USB-hub. I have a USB flash drive and a printer connected, and it works great. I tried plugging in a second USB flash drive:
- The drive is auto-mounted by the NAS, and i can see both drives in System Management --> USB Devices
- Both USB drives are browseable in the My Files filemanager
- Both drives are automatically shared through NFS
- Only the first USB drive is shared through CIFS/Samba
- Second USB drive doesn't show up in Account Management --> Network Shares, and you can't add it as a new share
Using more then one USB drive is perfectly doable, and I don't think you will see any problems if you use a powered hub for your non-powered hard drives. But if you want to set special share permissions, or to use CIFS/Samba for all drives, it seems you have to use fun_plug to set up these share yourself.