And remember, with very large drives, there is a 1000 program limit (may be 999) on the EHD, so if you record 1000 30minute programs and it only fills 1TB of the drive, that is all you can record until you delete something.
A 3,000MB drive recording a 2GB per hour could theoretically hold 1,500 1 hour programs, but there is that limit to how many programs you can record.
Perhaps Dish will raise that number if they can recognize a 3TB drive.
Then again if you record 2 hour movies, you could put 750 of them on that 3 TB drive and when it finally goes belly up - they are all gone at once!