Not helpful to your current problem, but personally I wouldn't bother with a USB connected drive on the HH. It's only USB2 so 60 MB/s in theory, but it performs muuuuuuch slower (see message 2 of HH5 local storage).
If you're sharing a 1TB drive it would suggest you want to use it for more than the odd file, so I'd find a NAS you can connect to the hub GE LAN ports.
After researching this issue for over a day. I solved it for me as follows.:-
I checked the BT Hub 5 page normally 192.168.1.254 BT Hub 5. Looked at the connect devices on the USB port it says my disk drive is connected but also says (file type not supported)
Despite trying to use it via the group share options it would not allow access to the USB device connected to it. This is most likely due to the file type not supported issues.
It would seem based on my research that this has been a long standing issue for all BT Hubs. It looks like the issue is one or two problems the size of the drive) mines 3 terabyte’s) or it can’t read either NTFS or or FAT32. I think this is a router software issue and BT need to resolve it.
However the WORK AROUND options are:-
I’ve not worked out how you can share this drive with other computers on the network and if anyone has any suggestions I would be most grateful to hear them. I will report back if I manage to solve the other part of this issue