How to delete info from a connected device on mac even system information?
By any chance do you mean delete everything on the MyBook Studio drive?

If so open Disk Utility, select the drive in the left-hand pane and then click the "Erase" tab, select the format you want (best bet: Mac OS Extended (Journaled)) and click erase.

Everything (and I mean everything) on that drive will vanish in a cloud of unseen bytes.