I use a 2TB Seagate Wireless drive. The iOS app called "Seagate Wireless" lets you set up the wifi password for the drive and you can get the drive to access another wifi network so as you can connect to the Seagate Wireless drive and still have wifi Internet access. The drive can also sync your Dropbox account to it so you have offline access to your Dropbox files if you are away from wifi. Even if you are away from wifi, you can access the wifi drive, you just don't get Internet access via it since it is not on wifi.
Via the app, I also enable my iOS photos from my iPhone and iPad to backup to the drive. This ensured I had a backup while travelling and was great in case I lost a device.
I use Handbreak to convert video to iPad format, just using the inbuilt settings, then I transfer the videos to our iPads and excess videos to the drive. This way, I can transfer excess videos to the iPads after we have watched the first lot of videos that are installed. This drive is USB 3, so transfer to the drive is reasonably fast. Wifi transfer to the iPads is not very fast, but you can set off a couple of files to transfer while watching a different video in the same app or while you go and shower. You do need to remain in the app that you are transferring to for the transfers to complete when you are transferring large files. I was using OPlayer for video watching and it was fine, but lately I have started using Infuse Player Pro and find it graphically pleasing, it also plays unconverted video more smoothly so I'm happy with it. I would still recommend converting to iPad format though since you don't need a higher quality for iPad viewing, you will have smaller file sizes and therefore fit more files and also less transfer times.
A bit of a side note or 'how to' just because this tripped me up the first time....I use the Settings app in the iPad to connect to the wifi on the Seagate Wireless drive....then open my player of choice and then connect to the drive which appears in the video app. OPlayer asks for a username and password and I leave both blank, you already used the wifi password to connect to the drive and there is no separate username and password once you are connected.