Seagate FreeAgent, 500 GB p/n 9nk2al-510 stopped working

There's not much troubelshooting you can do with a failed external disk. power off PC and hd and then disconnect all cables, wait 30 secs, power the PC on, let it boot to the OS, power the hard drive on and then connect. If the drive shows up but you still cannot access it or get weird property readings, it generally means the hd controller has failed. If the data's worth more than $1000, you can take it to a data recovery group like Ontrak or Cherry Systems, but otherwise, just get a new hd and restore whatever backups you have.