I thought I'd share my story with a WD5000LPVT that I had a hell of a time getting to work.
I had tried 4 different computers trying to flash the 500GB security sectors files, and it would never stick properly, never would be detected by the Xbox 360. After a lot of back and forth, and referencing dumped firmwares from a WD5000BPVT that did flash successfully and worked in an Xbox 360, I discovered something interesting.
Hddhackr 1.40 seemed to not be writing the security sectors properly, as when I would reflashed the 500GB security sectors, the resulting undo.bin (backup) file was a little bit messed up. I'll upload a zip file with the firmware from the LPVT (badflash.bin) and the good working firmware from the BPVT (BPVTFIRM.BIN).
The most obvious sign of the bad flash was directly seen in Hddhackr as well, the serial number of the flashed drive would show as a slightly truncated serial number of the original firmware, instead of the 500GB security sector's serial of J2260056GD9SXC.
I took a complete shot in the dark and decided I would just flash the BPVT firmware over to the LPVT, and this luckily worked. My only guess as to why that worked instead of just flashing the security sector is that the code that flashes the whole firmware versus just the security sectors worked differently on this drive, enough so to write everything properly and we get the correct serial and the drive showing up in the Xbox 360.
My hunch is that this is similar to what may be happening with other drives, like the WD5000LPLX that is mentioned as troublesome on the main post.
I'm not going to encourage anyone with trouble flashing their 500GB WD drives to use the BPVT firmware I'm sharing here, as I have no idea if the firmwares of flashed compatible drives should be identical for same drive sizes, but if you're willing to take a risk, you at least know my experience here.