A zif to usb adapter would certainly make the HDD visible to a computer.
I found this article:
For certain iPod models—such as the iPod classic and iPod nano—there’s an easy way to copy all the content from your iPod to your Mac. Start by connecting your iPod to your Mac and immediately holding down the Command and Option keys until it’s mounted. (This prevents iTunes from syncing it automatically.) Next, download Titanium’s Software’s free utility Onyx. Launch the program, click on the Parameters icon in the toolbar, then on the Finder tab. In the Misc Options section, check Show Hidden Files And Folders, then click Continue in the warning window that appears. The Finder will restart, and you’ll be able to see hidden items.
Double-click on your iPod’s icon on the desktop, then navigate to the iPod_Control folder, then the Music folder inside that. Here you’ll find a number of folders named F00, F01, F02, and so on. Each of these folders contains media files—music, TV shows, movies—but they aren’t named the same way you’d see them in your iTunes Media folder. Which makes it hard to tell what’s what.
Although you may be able to narrow down items by their cover art (if it shows up), and determine songs or videos by playing them in the Finder, it’s more realistic to think of restoring your content via this method as an all-or-nothing endeavor. Drag all of the folders to your iTunes library, and once the files have copied over, iTunes will display the correct tags, and you’ll have all your files back. If what’s on your iPod is only part of your library, create a new playlist first and add the files to that playlist so you can sift through it and delete the files you already have.
Unfortunately, this method doesn’t work with everything. iOS devices don’t mount on the desktop, and therefore aren’t accessible in the same way. If you need to copy files off an iPod touch, iPhone, or iPad—or just want better control over the files—there is software that can do the job.''
I don't know if you have an apple computer, the article assumes that.
There are programs like sharepod for windows, but again I think it only works with an iPod rather than just the HDD.
So really you have 2 choices, buy the adapter and have a tinker and see if you can get the data off some how. Or put the drive back in the iPod...