It's not fair, that's for sure, but on the other hand they will benefit from the format of the disk as all the down time they have right now is much worse then the re-format of the disk...
Just let them know in any way you can - an announcement on the Forum, on the Main page -heliohost - and a mass email are enough to let everyone know that something will happen in the next couple of days. I think that the majority of them are already visiting the main heliohost page everyday to check out what are the problems and so they would read the anouncement and would do the back-up...The ones who do not check the main page or the forum regularly just risk to miss some important announcement and thus it is their own failure...
In the end it wil help all of them more then having a page that is 75% of the time down, because of an issue that could be solved by re-formating the disk.
It's cruel, maybe, but if it helps it is a good solution. But it is only my personal opinion. Good luck.
Srry for not such a great english :-)