Moving data on a raw device with a mounted partition

[Ben Scott]

I need to be able to move data around a disk that has mounted
partitions on Linux. I am not touching data inside the mounted
partitions or the MBR, only the free space around it or unmounted
partitions. So, for example, I would need access to "/dev/sdb" while
"/dev/sdb1" is mounted.

Currently I am using libparted functions but the move is giving me
corruption when the disk is busy. Would the system call open(pathname,
flags) work here? If I use open() are there any major gotchas I should
be aware of? Also, what flags would be a good idea? I'll need
O_LARGEFILE and O_NOATIME but I am wondering about O_DIRECT for
example.

Thank you.