Some Dell computers will fail to boot and show an amber light at the start button. This happened a few times to my Optiplex 745 and every time it was because I had an external USB hard disk plugged into the front USB port. When I unplugged the ext. USB drive the computer started fine. This computer will not start when there is an external USB disk plugged in.
On my other Dell WS 490 the problem is a little different. I can use the external USB drive only with SATA power ( ~ 0.5 Amp) and not with the AC power adapter plugged into the ext. drive (2.5" drive).
I just have to remember these things when I start these 2 Dell computers.