Computer anomaly of the month!
Built a new HTPC back in December and it's worked absolutely flawlessly. 1TB of space and I can't complain. The OS/music/pictures all resided on a 200GB disk from my old media center. I decided to put in a new disk since that 200GB one was getting old. Grabbed a new 250GB off newegg, installed it, fired up the ubuntu live cd and used some dd to mirror the disk right over, precise copy. Friggen. Awesome. Booted up the new disk perfectly and started watchin' TV again. All was good until the tuner failed...odd. Checked connections, reseated everything, checked for something that got knocked loose during the HDD swap (case is a tight squeeze). Nothing would work. So I grabbed my spare USB tuner from my laptop and put that in to hold everything over...that worked for two days until THAT failed. Neither set of tuners work. "The device failed to start" boasts XP. Well...fantastic. Trying everything in my power to get it working I finally give up and toss the old HDD back in. Now it works fine...but I'm back on the old HDD. The only thing I can remotely fathom is the new HDD draws a slight bit more current, throwing off the balance of the meager 300w PSU (...advertised as 300, side says 185w...). Even that sounds wrong. Guess I'll try dd again and hope it works.

are you streaming while watching? it might be pulling the 5v power rail too low. have you tried running it through a powered usb hub?

When I'm trying to use the tuner MCE saves it to the disk for the rewindability feature and then shoves it up to the screen. The external tuners have their own wall wart, the other tuners are the Happauge WinTV 500 (PCI).

it's possible that the new hard drive is electrically noisy...does it act the same within an enclosure/run off of an enclosure's power supply?

Eh now that might make the difference. I don't have a separate enclosure I can use but I can use a separate PSU I could hook it too and have it outside of the main computer chassis.

The grounds should be tied together though. I would maybe run an alligator clip from your separate PSU to the chassis of your box. Or perhaps something more permanent.

Bah well the same problem is happening with the old drive...only one tuner (of the two on a single chip (one is PCI, other is USB...)) initializes properly. So I've got dual tuners but I've got 4 hooked up...bah!

What's the dmesg output?

Computer runs XP MCE and I know for a fact linux doesn't play nice with the USB tuners (lack of drivers), although it should with the PCI ones.