iMacs work wonderfully with external optical drives. I have one on both my iMac and Mac mini. The fusion drive is basically both a physical hard drive as well as a solid state hard drive combined. The fusion drive will work in anything... in fact I see Seagate sells a solid state/physical hard drive you can install to replace your current physical hard drive. (a physical hard drive/disk is one that has the spinning platters and a solid state drive is one that has no moving parts) So to answer your question, the hard drive has nothing to do with the optical drive and they will both work on your iMac.
You can try a reboot. Press and hold the 'Home" button and the "ON/OFF" button at the same time and hold them until it restarts and you see the Apple Logo appear on the screen... then you can let go of the two buttons. This should "reset" your iPad and it should work properly.
I am thinking you have no room to run even the operating system your startup disk is so full. I am not sure by your question if you are running a Mac or Windows computer. Either way, you MAY be able to use your original disc (DVD) that came with your computer and boot from it. This way you are not using the hard drive to boot to. Then you can either wipe out your hard disk and reinstall the original Operating System or there may be some way to delete some of the data or programs and restart your system up afterward. If that is not possible, you may want to either replace your hard drive with a larger one and re-install your operating system from your original disc. Then re-install only the stuff you need and not overload your hard drive again. Another option is to remove your hard drive, purchase a disk transfer cable, hook it up to another computer then delete some of the junk off of it so your operating system can have room to load. If you go the route of putting in a larger hard drive, you can...
You most likely have a bad "eye", otherwise known as the laser lens or reader. Unfortunately, XBox optical drives are "married" to the motherboard, so you cannot simply replace the drive with another. I invented a workaround though... If you can find an old XBox with a good optical disc drive; you can remove the "eye" or laser lens and swap it with the one in your XBox. This leaves the coded optical disc drive hardware in your XBox so it will operate just fine. Heaven forbid Microsoft letting you simply replace an optical drive.
I had this very issue and spent six hours on the phone with Apple and they couldn't fix it ...but here is what I discovered and it worked to clear up the issue. First, see if the iPod works on a different computer without automatically going to "OK to Disconnect". If it doesn't, it may have issues. If it does, you need to perform a "Restore" (not a reset) on your 6th Gen Nano. This will wipe out any tunes on the iPod, but you said you don't like the ones on it anyway. Once the Restore is complete, first and foremost, tell it NOT to automatically sync to iTunes on the registration screen that pops up immediately after the restore by unchecking the related box. Then, be sure to Rename the iPod. That's the fix that worked for me, so hope you have the same results. For instructions on how to perform a Restore, go to www.apple.com/support and type in "Restore Nano 6th gen" in the Support page Search box in the upper right corner of the screen. That will bring up instructions on how to do the Restore. Good Luck!
First, try unplugging the Mac from the wall for two minutes and plug it back in. Try restarting. If that doesn't work, attempt booting in "Safe Mode" by booting up and as soon as you hear the startup chime, press and hold the "Shift" key until you see the Grey Apple and Spinning Gear. Then let go. Starting in Safe Mode fixes a couple things, so try the unplug first. Works so many times after power has gone out. Another possible fix is resetting the PRAM.
There are several reasons an iPod won't show up in iTunes. And not so uncommon. A great place to start is on www.apple.com/support - there you can type yor question in the support "Search" box and it's fairly easy to find Apple's iTunes troubleshooting guide as to why your iPod won't show up. With a little luck, you might find one of their suggestions helpful and once your iPod shows up in iTunes, you can get on with the " Restore" process which will wipe and re-install the iPod's operating system software. Oh... and will remove all your songs from your iPod as well. Best wishes!