stuck in network booting mode

I'm stuck in network start-up mode. I’ve recently had my Mac repaired so I guess they’ve used network start-up there to diagnose the drive. The funny thing is that first the computer gets locked at the spinning wheel (or the screen is just grey); but once I disconnect the cable to my external devices (modem and x-tra HD) the computer suddenly behaves normal again and continues with a normal start up. I’ve tried all the tricks in the book, including resetting PRAM, DiskWarrior etc. But the problem just won’t go away. Can I somehow erase some settings which might have gotten stuck in my machine?

/Svante

iMac 8,1
OS X 10.5.8

Did you reset the startup disk from System Preferences, Startup Disk? Unless you reset it from there it will keep looking for a network boot.

yes I've tried it several times

Yes. I did so several times and also clicked on the locker to make it stay that way. Be lo and behold: the system unlocks the locker and I'm back on square 1.

Now I discovered what's interfering with the start up. It turns out that it's Time Machine on my external hard disk. If I boot with my external disk disconnected booting is fine. But I still don't now how to solve the issue