Startup Programs

I was wondering if there is any way to reroute a computer to select its hard drive's startup rather than the disk's startup.
In other words, Using the computer's startup other than the disk's.

Without acess to the rom files, there is not. If you do however have acess to them, you can edit this part of the shell to acheive what you want.

-- Then run the user created startup, from the disks or the root
local sUserStartup = shell.resolveProgram( "/startup" )
for n,sSide in pairs( peripheral.getNames() ) do
  if disk.isPresent( sSide ) and disk.hasData( sSide ) then
   local sDiskStartup = shell.resolveProgram( fs.combine(disk.getMountPath( sSide ), "startup") )
   if sDiskStartup then
    sUserStartup = sDiskStartup
    break
   end
  end
end

if sUserStartup then
  shell.run( sUserStartup )
end

Well thanks.
Now to figure out how to access them.

You can do something similar if you have access to the disk, just stick this at the start of the disk's startup script:

if fs.exists("startup") then shell.run("startup") end

I had a setup where I placed a disk drive next to a computer containing a disk with a script that programmed new turtles. Obviously I didn't want the computer to configure itself as a turtle, but since I didn't want to move the disk around every time I wanted to change its contents, I simply rigged the code as above. Well, actually I rigged it so that it detected whether the system was a turtle or not, but you get the idea.

Of course, shortly after I implemented that the MineCraft server started stalling every few hours or so. I never fully diagnosed the issue, but I did eventually narrow it down to that system. Beats me whether it was the usage of my disk drive or not.