Is everything on the USB or do you have a microSD as well? Personally I use a use a 32GB class 10 MicroSD with swapfile, timeshift and epg setup on it and just use a hdd for recordings.
If you intend to put everything on the HDD then I would convert it to Ext4, switch off the ZG, swap drives over and then power it up.
You will then have to setup the swap file etc again but you won't need to initialise the drive.