How can I tell mac to stop trying to connect "idisk" server? 10.10.5 and 10.11.6 on same network, Imac and macbook pro. I get a re-ocurring popup that says,  "cannot connect to idisk" popup notification. It keeps returning even though I select "cancel". It holds my system hostage, not allowing other apps to work until I (again, and again) try to click it away by pressing cancel. It goes away for a while, but eventually returns.
My own experience is with PCs, but I'll give this a shot.

From what I've been able to discern, iDrive is an online backup program. You don't say, but I'm assuming you have an iDrive account, and it keeps trying to connect. I've seen similar behavior with GoogleSync, when it checks files and brings my own system to a standstill until it's finished. 

I checked, and Macs have an equivalent to Task Manager, the Force Quit Applications Manager. Command+ Option+ Escape will bring this dialog up. Search for the iDrive application in the list of apps. There should be an option to click on the list of apps (process name), and it should sort -- I'm basing this on PC experience. Scroll through the list for the iDisk app. Click on it to select, and the QUIT button is on the upper left corner. This should terminate the application. This should free up your machine to do other things.

Once you are done, however, this is a temporary solution. The next time you boot up, you will have the same problem. You need to address WHY the app is having problems. I have not used iDrive, but assume it's subscription. Is your account active? If not, you should uninstall the app. If you are a current and active subscriber, you might then need to call the company to find out why you aren't able to log in. I am assuming it worked before without issues and without bringing your system to a standstill. Assuming you are subscribed and want to keep the app, there may be an option to repair the application; not all software has that option, but the better ones do. Otherwise, barring a call to the company, you might need to uninstall and re-install. Macs are pretty stable machines, but sometimes an application does get corrupted.

The steps above should help you get control of your Mac. Again, then you need to deal with the iDrive program and why it's misbehaving. Good luck!