I am having some trouble accessing my mybook and I was thinking about doing a hard reset on it. I have the ssh, cups and a few other hacks and tweaks, oh and loads of data on there, do you know how much I would loose by doing it?
Depends on what you mean by "Hard Reset"… If you use the info in the "Backup/Restore" forum and re-image your system partitions, then you will lose all of your hacks and tweaks, but it can be done without losing any data.
There's also a thread (http://mybookworld.wikidot.com/forum/t-67423/easy-revert-to-original-firmware) on that forum on how to use the "upgrade firmware" button in the web interface to basically re-flash your system partitions, but again, you'll lose your hacks and tweaks. This second procedure is probably the cleanest way to do what you want to do. It'll bring the MBWE back to factory software, keeping your userdata. It's easy to reapply the SSH hack, and from there you'll have to redo the rest of your hacks, but you won't have to re-setup all of your shares/users/etc.
Hope this answers your question.