It could be the ROM waiting for the HDD to spin up. Some don't wait as long. When the computer powered up and you are looking at the purple "insert disk" screen, press ctrl+amiga+amiga (soft reset) and the machine should boot.
Also, some drives power down as you hold reset. To stop this, cut the 1st (usually red) cable. It is not needed on Amigas and will stop the drive motor powering down. There is no advantage or disadvantage to keeping the cable in tact if the motor does not power down, so try my first point before getting your cutters out.
