I didn't find a thread concerning hard drives. I saw a couple mentions and some got told it wasn't possible.

I was successful...
Western Digital 600gb HDD
FAT32 Format
Power Inverter

First of all make sure that you get a hard drive that is not powered through usb, the fit's usb is not powered.
Make sure the drive is formatted in FAT32. If anyone is wondering how to format a 600gb hard drive in windows 2000 and up, you have to do it 32gb at a time. This isn't a problem for me considering I haven't even filled 8gb yet.
Right click on My Computer, select Manage, Storage, then Disk Management. After 32gb FAT32 is no longer an option.
The power inverter is required to power the hard drive. I plan to permanently install it without having to use the 12v in the front of the car eventually.

I have not tested if all 600gb will work. I don't have enough music for that yet lol

 

I think if you use Disk Management, it is only going to create a 32GB partition and leave the rest unallocated. You can use a third-party program like this one that will be able to format more than 32GB.

 

Im not too sure up just a suggestion, some of the 2.5" hhd enclosure that are USB powered have a usb cable that looks like a Y, one for data transfer and the other for power, so you might be able to use that instead of an inverter.

 

Like I said in the original post, the usb in the Fit is not powered.

I used disk management straight from windows and kept making 32GB partitions.