A 600 won´t boot

[Zwerg]

Hi,
i am a complete newbie to Amiga but was asked for help by a friend of mine. She bought an used A 600 on ebay, to play the games of her childhood.
Now, after switching on the screen appears to insert a workbench disk and everything looks fine so far. However, none of the disks she got would work, the drive is working for a few seconds and the screen reappears. I figured the drive might be broken and she got an additional external one, but again with the same result.

I searched the web a found several posts concerning colored screens at bootup indicating different errors. But in this cas it´s the boot screen. I tried different disk drives and different disks. I would not expect that both drives are broken and neither should all the disks she got from different people be bad.

What else could it be? Do we miss some basics (after all, we are newbies) or is something broken?

Thanks a lot for your help.

[Zwerg]

First, thanks for answering, your help is greatly appreciated.

@Boot_WB:
the disks were both original disk for workbench 2.x and some bootable game (Lemmings I think) and copies for workbench 1.3.
These disks came from at least two different sources on ebay. I cannot be sure that they are working but that would really be bad luck if ALL disks she bought were broken.

@countzero
I just plugged the external drive in the slot on the back and when i inserted a disk i got the same reaction as with the internal drive: the ´insert disk´ logo disappears for a few seconds and then reappears.
As I said, i am newbie to Amiga and i am not aware how to mount a drive before boot.