But I don't think that's the answer you're looking for.
Before you donate the drive, however, you need to make sure that there's nothing of a private nature on it. If there are files that might embarrass you, or that might prove valuable to identity thieves, you need to properly destroy them.
You can also make use of the drive yourself. If your computer has an extra drive bay (very likely if you have a desktop; very unlikely if you have a laptop), you can install an old internal drive and gain a bit more storage. Assuming that the drive connects via today's standard SATA interface, this is a very simple operation. Older drives may use an IDE interface, which your PC's motherboard might not support.
Finally, you can turn an internal drive into an external one. All you need is a USB enclosure. These generally cost about $20, but I've seen them for as little as $10.