Find A Waking Process

Determine which process is responsible for waking a disk

While adding such functionality is infeasible given the App Store policy regarding administrator privileges, it is still possible to monitor disk activity using fs_usage via the Terminal. After executing it with administrator privileges, fs_usage displays all file activity for all processes. Then, using grep, the output can be filtered to display only the file path of interest. For example, to monitor activity for an external device named "My External":

sudo fs_usage | grep /Volumes/My\ External
NOTE: All spaces must be preceded with a backslash (My\ External rather than just My External).

Once executed, the fs_usage command returns activity as it happens.
As a process accesses the specified device, five columns of information are returned, only three of which are mainly of interest:

The first column shows the time of the disk activity, the third column shows the path that was accessed (/Volume/My External/install.img), and the fifth column shows the process that accessed it (Finder).

The combination of these commands should help shed some light as to which processes are responsible for waking disks from sleep.