Try connecting the iPod to iTunes in disk mode using the instructions in this article. If you can get it to appear in iTunes, you'll want to restore it to factory settings.
Exactly the same happened to me last night - 120GB classic, plugged it in as normal to synch up podcasts and it scrubbed everything. I've reset the computer and ipod, I've restored to factory settings, I've left it overnight synching - but all to no avail. Any further suggestions?
As it stands, it seems to get the content copied over but then fails to finish the synch. It gets stuck in a loop of "finishing synch" messages and never actually finishes, and that includes having left it all night. Having had to unplug it (after it had been trying for 8 hours to finish the synch of 30GB of music and podcasts) it's got all the content on the device but as "other" content, inaccessible with only 245 tracks (out of 5,000+) and 12 podcasts (out of 60+) being visible.
It could be an indication or a damaged or dying hard drive. I would recommend doing a disk diagnostic on the iPod's HD using the instructions given by turingtest2 in this older thread. He also covers what the resulting numbers indicate.
If necessary, post the resultings numbers and we can verify what the next steps to take.
The numbers are;
pending sectors: 40
poweron hours: 1125
temp: current 27c
temp: min 5c
temp: max 54c
Any suggestions gratefully received - I've tried restore, I've tried formatting (high and low level), I've tried the scan and fix bad blocks option - nothing helps. I can get up to around 35GB onto the drive, any more and it fails (stops sync and resets ipod).
Worth investigating getting a replacement drive?