I have a seagate network drive connected to my home network, and i can accesses this either at home or away (3G or wifi) on my iphone/ipad using either thier app or logging in via the web.
You can use them with a Surface RT/Pro/Windows 8 convertible but I don't think you can use them with iPads or Galaxy Tabs as they don't have USB ports. I think out of all the tablets around the iPad is the most limited in capability really.
They can be used with Android tablets that support USB On The Go, you just need a short cable to plug them in. The Nexus tablets, for example, support this.