Can you set up the hard disk as a NAS, ie leave connected to a raspberry pi or directly to your router? You could then mount to the network drive. A flash drive is still advisable for ABM and EPG data and picons due to the high number of data writes to the drive (long term may damage a traditional disk) and because the box will be able to access it quicker. Your networked HDD will still be fine to record onto if the shares are set up correctly.
Alternatively if you're using a desktop hard drive (ie with a mains plug) you should be ok to put a usb hub in the back and use both from the same port. I wouldn't do this with a non-powered hard disk though as the current demand on the back port may be too high (unless you use a powered usb hub).
Not sure about using your HDD without formatting, I suppose it depends how it was formatted originally; as a linux OS user all my drives get formatted Ext4 apart from the USB sticks used for flashing of course!
For mounting it's blue button, then mounts and you'll see the various options.
Don't forget you'll also need to change save locations so recorded stuff goes to the HDD and epg, picons etc go to the USB