Gx280 SD second hard drive

no you cannot, i would be very surprised if it fits at all

If it's anything like the GX240 SD case then by all means YES you can. They sure look similar.

My GX240 SD currently has a 40GB drive in the HD bay and a 120GB drive in floppy drive bay and has been running like that for months. (since I got it)

Floppies are done anyway so this mod might work for you too.

Mount the floppy drive's rails on the HD. Adjust the rails so they stick out towards the front as much as possible, it's a snug fit.
I then modified the case by removing the floppy eject button, trimming it shorter and gluing in place flush with the front of the case so there wouldn't just be a hole there.
If you don't remove the floppy drive eject button, the HD will push it so it sticks out of the case by about half an inch, but it will flex to allow you to do a test fit prior to commiting to the mod.
I used a "Y" adapter for the power connections and the HD cable plugged right in.
I then removed the floppy drive cable entirely.
EDIT
Keep in mind that you will be able to see the end of the HD in the floppy drive slot.
For HDs that are black across the front it is a non-issue, but if it is unsightly you can always remove stickers or put a strip of electrical tape across the front of your HD. It sounds strange, but it looks just fine, I assure you.

EDIT AGAIN
On further inspection it appears that there are some models of GX280 small desktops without a floppy drive installed. It should still work, but you'd have to obtain the floppy drive rails (not HD rails) and possibly a long ribbon cable with the additional connector on it if your machine doesn't have one.
More work, but still entirely possible.
(I'd love to find a floppy-less faceplate for my GX240, so I'm off to look for one!)

Good luck!

Keep in mind that this is the Small Desktop case I'm refering to, not the Small Form Factor or the Tower

YET ANOTHER EDIT (they don't make this easy)

Okay this is assuming you are running the OLD style GX280 case and not the NEW style!!

Happy days I new it look possible. Which type of hard drive did you use and ridden my 280sd does not have the floppy drive but does have the ridden and power cable for a flap also my machine has the cover on the front to cover the whole because I do not have flap drive.  Thank you very much for your corresponds hope you will reply so I can get started on this project 

 

Yours sincerely lexor

 

Dell Optiplex

Gx280 SD

2.8 GHz

40.HD

1U slim line case

If you don't have a ribbon cable with two drive connectors on it then you'd probably be best off getting a used cable from eBay or some other online auction site. It will likely be listed for your model.
Generic ones probably won't be the right length.

The Optiplex line utilized this Small Desktop form factor case for a bunch of models, so you shouldn't have much trouble finding a used cable. Same goes for floppy drive rails.

For the power connection you could use a floppy-to-hard drive power adapter as well. I happened to have the hard drive Y-adapter handy in my parts box.

My hard drive I'm using is a common maxtor 120GB drive. The BIOS recognized it no problem.