Slider auto goes to "Off"

Question

I have a large HD hooked via firewire into a headless Mac Mini.
Using Airport wireless, I've been able to back up 3 portables (plus the Mini itself) to this drive.
However, I can no longer back up our Macbook Pro. Every time I attempt to choose the shared drive, the on-off slider switches automatically to the "off" position.
Any thoughts?

Answer 1

Unless you use a 3rd party tool to change the backup interval as well as the backup time, I don't know how you can prevent 2 or more of your Macs from running a Time Machine backup at the same time. If the users are also doing other network traffic intensive activity like watching a video or listening to streaming music, it is likely your four Mac can loose the connection to Time Capsule or your backup drive. Assuming your portables are connected to the backup drive wirelessly, the bandwidth available is even lower (54 Mbits per sec vs 100 Mbps if connected by wire to TC).
In addition, just because Apple say TC supports up to (Is it 6?) so many computers backing up to it doesn't mean it's advisable. On a PC, I know that if I am defragging a hard disk, another disk-intensive operation (like installing a software update) would be painfully slow.
Checking if this is indeed the cause of your problem can take some work, but a simple tool can help you avoid this problem altogether. I use a free tool called Time Machine Editor..
http://timesoftware.free.fr/timemachineeditor/
which allows me to set the backup interval and time. If you use this tool to separate the backup time for each of your computers by 10 minutes, you will have 6 non-overlapping back-up periods, even for up to 6 computers to backup to the same TC with no interference from the others. This is only if Time Machine is allowed to run hourly on each computer. If you backup less frequently, you have a longer non-overlapping (only one Mac backing up to TC) backup period. e.g. If every computer on the TC backs up every 2 hours, 12 different computers each gets 10 minutes non-overlapping backup period, or 6 different computers each gets 20 minutes of non-overlapping backup.
Apple sells the hourly backup as a way to recover from user errors that completely wipes out an importance file. However, there is a price to pay for that in terms of increase usage in TC bandwidth. I've read from other users in the Time Machine forum who recommends a lower backup frequency.