I have a Seagate 500GB external hard drive that has been put in a new casing after some wires in the original came loose. I only got it back two days ago.

So today I plugged it into my macbook and everything was fine, and then suddenly the warning/error window pops up saying that it's not been ejected safely. I hadn't touched the acutal hard drive or the wire or anything. I thought it might've overheated or something so I unplugged it, left it for a few minutes and then plugged it back in and nothing happened. The light on the actual hard drive's turning on but that's it, the laptop's not registering it. I've tried it on my mums windows laptop and it does the same. It lights up but no icon appears on the desktop.

I've also looked for it in disk utility and there's nothing there either.

Bad repair?

Try to download a free recovery program on your drive and let it scan your hard drive (have some hope it will locate the ex-HDD).

Not good. please check "About this mac" Located under the apple. Run the system report and check to see if it is connected via the hardware. You state that you connected it to a window's machine so I will go with a USB connection. Just look at USB under hardware and see if you spot your HD. (Hint. Look for Seagate).
 I doubt that it will show up from the sounds of your problem. Send it back for a new enclosure, or get a new enclosure and drop the drive in. I do think that your data is intact and recoverable, Just not in that inclosure.
    There is a possability that it will show up. This indicated that the boot block on the drive is corrupt. In that case you need some serious diagnostic software. Tech tool pro (TT6) comes to mind. TT6 will mount the drive if it can be seen by the Mac. Copy what software you can get off of it and reformat the drive. You should be good to go, but I would **not** trust that inclosure. You can buy a nonane enclosure for $20.00 on ebay these days
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