I've replaced many over the years, but the crappiest moment was walking into the house to this nasty stink.
It was a long time ago when drives were just getting to the 60 gig mark and gigs were expensive. I had a freeBSD fileserver with 3 or 4 smallish drives in it, and the power supply went wild. Overvoltage popped every chip on all the controllers, the NIC, motherboard. Napster music = gone. my burned CDs = gone.
I was bench testing a system once when the drive's controller card flared up. It actually caught fire, but it was just a quick flare up so didn't catch anything else.
Just the other week I spent the better part of 2 days trying to recover data off my girlfriend's failing drive onto a spare. I had backups and everything, but figured it'd be quicker to just do a quick dd or partition copy as the drive was still mostly functional. But every tool kept dying out and wouldn't force through the last block read errors on the disk except good ol' cp. By the time I got down to just running a cp command on everything it had become a vendetta against the drive, but at least during the process I managed to set up a nice customized hirens thumb drive and Linux rescue thumb drive. And it preserved all the NTFS permissions to boot.
My friend just lost his second main hard drive in two years, but instead of buying a new one or going SSD he decided to just bin an entire custom built i5-2500k gaming system. I'm trying to get him to at least sell me the remains, but I'm thinking he's going to just store them or trash them outright.
Come to think of it, most of my drive failures in the past 3 years or so have all been Segates. Though I have a pair of Segate 1TB drives in my main PC now that have been chumming along flawlessly, though older than the firmware fiasco a few years ago. I also have a OCZ Vertex 2 that hasn't given me any problems, but I'm on my second Corsair M300 which is still glitchy as hell (using the 2nd Gen SandForce I think). My home server is currently running four 2TB Hitachi drives in a RAID0 that replaced some Segates (one failed out of those when it was a RAID5). Oh so fast for non-SAS spinning disk and it's playing with fire, but did I mention it's fast? I need to install more gigabit NICs and team them since it's limited by network connections at the moment. Doesn't really bug me all that much about the potential for data loss -- I back up onsite to external USB enclosures, between my other computers, and to LTO3 tapes, and offsite for important stuff (photos, homemade vids, important documents, etc).