1. Be sure you have following options checked in your Ubuntu kernel:
File systems -> Miscellaneous filesystems -> UFS file system support (read only) (UFS_FS)
File systems -> Partition Types -> PC BIOS (MSDOS partition tables) support (MSDOS_PARTITION) -> BSD disklabel (FreeBSD partition tables) support (BSD_DISKLABEL)
Default kernel for Ubuntu has already this options.
2. After rebooting type in console:
dmesg | grep bsd
You shoud receive similar output:
[ 2.108529] sda4: <bsd: sda8 sda9>
It means that you have Freebsd installed on sda4 partition, and sda8 and sda9 are so-called “slices”
3. Use command similar to:
mount -t ufs -r -o ufstype=ufs2 /dev/sda8 /media/free
For older versions of Freebsd:
mount -t ufs -r -o ufstype=44bsd /dev/sda8 /media/free
mount -t ufs -r -o ufstype=ufs /dev/sda8 /media/free
might be more suitable.