I suggest that, if technically possible, that when a backup is taking place the forums be replaced, as seen from the outside, with a simple message that a backup is taking place and to come back in a few minutes. This would prevent members being able to log on, have some functions work and others not, producing uncertainties in the minds of the users as to whether it is the system or themselves that is the problem. After all, isn't in a concern here to avoid upset to members as much as possible?
If this is not technically feasible, would it be possible once one logs on to have a message warning of possible slowdowns?