Mounting external HFS+ device with truecrypt - read only file-system

I´m trying to mount my external HFS+ HDDs on my raspberry pi with raspbian "wheezy". The disks are encrypted with truecrypt so I mount them with the command truecrypt /dev/sda /media/usbdisk/disk1. But each time I try to write something to the disk, for example a new folder, it tells me
mkdir: cannot create directory 'newFolder': Read-only file system. I´ve tried to google that but could not find anything that solves the problem. I have already installed the packages hfsplus, hfsutils and hfsprogs. Does anyone know what I have to do to write something to the disks?