If I'm doing a passive task (i.e.: Generate & download a database dump), is ethical to pass that time as billable hours? I'm not working, but in that moment I can't do any other tasks for my work (suppose that my computer is busy). Instead, I can go to the supermarket or see cable tv).
Yes, it´s probably billable time but ...
Morally and depending on jurisdiction and contract, you have the obligation towards your client to make the best use of his paid time - going to the Supermarket/Watch may pose a problem. You normally also have the obligation to advise him to the best of your knowledge, so you should make suggestions how to make more effective use of your time.
I´d try one of the following approaches.
If it is only an occasional incident:
Plan your task so that your PC does the downloading/etc. when you wouldn´t use it anyways, don´t bill that time.
Occupy yourself with something at least remotely useful to the client, such as studying documentation, going over your planning or preparing status reports.
For work where you have to occasionally click/configure something and than wait again, I´d think about rounding up every phase where you have to do something to the next full quarter or so, but not bill the time where you where really doing nothing (watching TV for 2 hours without interruption)
If there are recurring tasks:
Get a different machine, that works in the background. The client may be willing to provide one if that saves a lot of billed hours.
Offer to automate those tasks, if possible, to have them run when you don´t need the machine.
Offer a different rate for low-intensity work, where you essentially rent out only your machine.
This may seem like you are loosing out on possible revenue that way, but being a good adviser as well as making effective use out of the client´s money will ensure you further and probably better paid contracts in the future!
I can't do any other tasks for my work
Then yes it is billable time.
Perhaps at a reduced rate due to it being "waiting time" as opposed to "active time". But that's your call. I'd have a hard time billing my standard rate if I was watching TV or running to the store merely because my system was busy with a task. But then, I'd have at least 2 systems if my work involved locking one up for a period of time.