[SOLVED] Can't access ext4 USB HDD

[freem@n]

it's most likely a permission problem. I just plugged a fresh ext4 drive in my Raspi just to test this and it looks like the default user xbian has no write permissions by default. probably easiest fix for this is to ssh into the Pi.. then do the following:

Terminal

cd /media
ls
###you should see a list of drives which is basically row of numbers and characters. for example my drive has the name 6a7bd302-7014-cf01-6079-d3027014cf01
###when you have found the correct name do this:
sudo chown -R xbian:staff 6a7bd302-7014-cf01-6079-d3027014cf01
#(change the name of the drive to your own). now you should be able to copy files to the drive

it seems like this worked at least for me. hopefully it helps in your case too