The internal drive is formatted in ext3. it actually has 3 partitions on it: a 180MB for temporary firmware storage, a 2GB for BD-Live storage and a 246GB for media.
If you remove the drive and attempt to read it, you won't get anywhere as it has an ATA user password on it. This is also the reason people reported it "killing" drives they tried to install. As soon as you install a drive and it asks to format it, it will lock it to the unit. I have a feeling each unit sets a unique password as well.
The way around this is to allow the drive to spin-up while installed in the BD590 and then with the power still on pull the SATA cable. Attach the still running drive to a PC and boot from UBCD. Use the ATAPWD program to "Set User Password" and then "Disable User Password". You can then remove the drive and freely browse the files in Linux.
If you reinstall the now unlocked drive in the BD590, it still works fine.
This isn't for the faint of heart. If you don't know what you're doing, don't try this! UBCD is easily found on the web along with instructions about using ATAPWD.
I have tried a few different drives in the unit. First off was a 500GB laptop drive which worked great. Then I tried a 7200rpm 1TB desktop drive. The unit will shut down as soon as it tries to spin it up. Too much power draw. I'm now using a WD "green" low power 1TB drive and it works great.
Just installing a new drive is a cake walk if you don't need to transfer the media from the old drive. Use a PC to remove all existing partitions, mount it, plug it in, turn it on, choose "Movie", and it will ask to format the new drive.