Black screen after HDD replacement

Obviously, not that easy. No startup chime and The screen is still dark.

You missed a connection somewhere or you connected the new drives up wrong.

This is the problem with DIY IMac repairs, there are lots of finicky connections that need to be made sure of as well as connecting up the correct connections to the various drives.

If you are not getting a startup chime, then something is definitely wrong, not connected or mis connected.

You'll need to tear down your iMac, again and this time watch the iFixit videos closely to make sure you have everything connected up properly and you didn't miss anything and reassemble the iMac.

If you installed non-Apple drives, be advised that once your iMac is running, again, you will need to install a software fan controller as no non-Apple drives have the need temp sensor connection, unless your model had a temp sensor connections that were taped to the exteriors of the drive enclosures.

Not Sure.

I just watched a video on the hard drive install process from OWC (macsales.com)

You may want to check this out and maybe consider ordering the special temp sensor cable needed for your non- Apple drives to work without resorting to using manual software fan controls (unless you use a Software fan control, already).

Make sure you didn't miss any of the screen connections and make sure that the video sync cable has locked into place.

Here is the video I watched.

Make sure you did every thing that is outlined in the video.

http://eshop.macsales.com/installvideos/imac_late27_2009_hd_h/

I just realized (it's early morning here for me, too!),

The Mac may not do anything or chime if the none of the drives have been formatted and an OS X has not been installed, but again, I am not sure about this either.

If you are using FW target disk mode from another Mac, you should be able to install a OS X system from that Mac to the new drives on the IMac.

Have you formatted the iMac's new drives and installed a version of OS X, yet?

That could be part of the issues.

'I was wearing anti-static gloves but had to take them off at some point to disconnect a cable that was finicky."

I do not know. As long as you touched something other than the iMac after taking off the gloves and you are not working on your iMac in a room with a carpeted floor, I do not see how you could've fried the logic board.

Again, not as simple as they show if something is done not right or missed or something else happened.

The problem is now you have to figure this out yourself, as of now, Apple or an Apple Store will NOT repair this now as there were no "officially" user serviceable items inside of an iMac for users to be replacing and you have broken into your iMac.

If you can't get this iMac working, then your only option is to find an Authorised Apple reseller/repair/service center to diagnose and give you the reason your iMac is now, no longer working after you made these alterations and have them do the work.

If you have a white, retail OS X 10.6 Snow Leopard disc, you should be able to use the other Mac's optical disc drive to format the iMac's drives and install OS X from that optical drive to the iMac's drives when in FW target disc mode.

If no startup chime, before you tear down again, try and SMC reset.

Intel-based Macs: Resetting the System Management Controller (SMC) - Apple Support

'Oh, additional data point: the iMac doesn't start up from an external drive."

If there is a full OS X system on your external hard drive, try holding down the option key at startup to bring up the startup Disc Manager.

This is about all I have for things to try at this point.

Good Luck to you!