This must be an identifier to separate similar configuration drives' different generations/versions. Usually bigger number refers to newer version (but not always, in example the new 7200.10 series has got digit "0" there, so they are starting from roots again).
* ST3400632A is 7200.8 series (16MB cache version, was short-time produced for retail kits only)
* ST3400633A is 7200.9 series
Note: This does not mean that all 7200.9 has "3" there. It is simply the following free number taken of the same configuration models (in example there has been more 250GB versions already, so the 250GB 7200.9 has got "4" there. Just counting up).
If your box states 632A-RK and the actual drive inside is 633A then very probably Seagate has changed the originally assemblied 7200.8 series drives with newer 7200.9 drive there, but the code on the box remained unchanged (may-be they have old stock of packaging boxes to use).
Thank you for the excellent information as always
I wanted to call up Seagate TS, but wasn't sure if they would give me this information over the phone. I bought the drive mid-July '06 and checked the warranty info (via the serial #) and it goes thru May 2011 ( lost 2 mos - usual Seagate DOM nonsense vs purchase date ).
I'm pretty sure this is still within the manufactring date of the 7200.9 series. Is there any other way to verify this also (firmware check etc in case Seagte TS doesn't confirm this info)? The drive was a pretty good price $120 no MIR from Fry's and Made in Singapore (my personal preference) vs Thailand or China (which seem to be newer manufacturing plants these days). I just wish I got it 2 weeks earlier, Seagate had a MIR promo on all their retail kits and this drive would have been $90 ... oh well, still a good price.