I've attached an external HD to the USB port on my Ultra Hub and turned on Samba.
I can access it using //192.168.1./Seagate_Expansion_1_521a in my browser, which means I can see what's there and download stuff, but how can I upload? Do I need to use ftp?
Or can I access it from Windows File Explorer? If so, how?
With the old Huawei modem, it appeared as a network drive called ATP if I recall....
You can use File Explorer.
I've done it successfully on Win10 as per:
Note: If it can't connect to the share, your PC may not be configured as a Samba Client - see my response at: