I wouldn't worry about TLER, unless you have a specific need for it, you probably don't need it. If it's having that much trouble with errors, it's time to replace the drive (or something) whether you have TLER or not. Unless you are running a mission critical server, a good backup is a better investment. Even if it is a mission critical system, a good backup is a better investment.
I agree with Tp4tissue about the reliability. I try and buy Seagate almost exclusively at this point. I use Hitachi on occasion, but I replace sooo many WD's on a day to day basis that I just can't stomach to buy them. Yes, I know many people have good luck with them, and some feel the same about Seagate. I used to buy from a guy who repaired drives, he had boxes of WD, and rarely any Seagate. There is only two WD drives I will buy and that is Blacks and Raptors, everything else I just shy (run) away from. Both are made of higher quality, and I have had Raptors hit by rain and survive (don't ask).
As others have mentioned, I would double check the warranty when buying through Ebay. Many companies will not accept a warranty on Ebay purchased items. This is likely true on drives as they are serial numbered and controlled.
As for the rest...
NAS boxes are extremely over-rated and over-priced. Build a mini-ITX server, you will do a lot better. All of these people complaining about drive speeds are fooling themselves if they own a NAS. Check your transfer rates people... Any cheap drive can saturate Wireless N and even a nice ATA will saturate gigabit, so why are you concerned with drive speeds on a NAS? My mini-ITX box can saturate Gigabit. I use my mini file server as if it was a local drive because it's fast enough. Try doing that with an old D-Link NAS. Also, if built right, it will be just as energy efficient. My file server with a single 2TB was using about 25 watts idle and about 35 during file transfers. Easily comparable to a NAS, and it's more capable as I can offload drive imaging, virus scans, torrents, large file transfers, web cam streams and more to it.
Years of Google testing has shown that spinning down a drive or leaving it running 24/7, has no effect on drive life. In fact Google found the only thing that really impacted drive lifespan (given good power and equal build quality) was vibration. Not even heat made a difference. Powering them down only saves about 5 watts, in the US that equates to about $5 if ran 24/7 over the course of an entire year. Needless to say, I quit babying my drives, except to make sure they were well powered, and bolted in well. I leave it running 24/7.
Instead of mirroring, invest in an online backup or something similar. Mirroring is not a backup, it's a fallback. If a drive fails, as a company you don't want to wait hours while a backup is restored. However, if a system is zapped by a bad power supply, lightening, etc... a mirrored drive is useless (seen it a few times), and worth even less than a usb external for backup. Unless it's a mission critical server, mirroring just adds complexity.
Personally, I have an agreement with a company 50 miles from me. I placed a 2TB low wattage mini-ITX system in their office and backup my stuff there. In exchange, I backup all of their files to my file server at the house on my 2TB file server. I use Crashplan's free software to handle the data transfer, it's fast, efficient and encrypts everything. Costs me nothing for offsite backup except the initial cost of building the server, of which, I only had to buy the drive.
