How badly do you want someone not to be able to recover the information? And how much time and resources do you have to do this?
Considering that you're dealing with over 2000 hard drives it will be rather difficult to wipe the hard drives so that the data is not recoverable unless you have a lot of time.
Several methods I can think of:
1) Quick format - Fairly fast but obviously not very effective
2) a. Overwriting - The DoD requirement for overwriting AIS storage media is to overwrite the data with a pattern, then its complement, then another pattern. Slow, but effective.
b. Overwriting - The DoD clearing requirement is to write any character to all data locations. Faster than the above but not as effective.
3) Hand-held magnet - Some people suggested this, but for this to be effective you either need to have a very powerful, and usually very heavy, magnet, or you need to take each hard drive apart and put the data platters in direct with a magnet.
4) Type I or Type II degausser - Very effective but also tend to be expensive, unless you want to rent one.
5) Physical damage to the hard drive - You said you need to replace the hard drive after wiping the data so you would need to damage the data platters in some way. Time consuming.
