Ironically, a disk I recently added was in fact one of the two spare disks my dad had in case one died (he held on to the larger one).
Update: Windows just presented me with this:
Delayed Write Failed : Windows was unable to save all the data for the file \Device\HarddiskVolume1\Program Files\WindowsUpdate\wuaudnld.tmp\cabs\co
m_microsoft.894320_W2K_SP5_x86_WinSE_154 269\Windows2000-KB894320-x86-ENU.EXE. The data has been lost. This error may be caused by a failure of your computer hardware or network connection. Please try to save this file elsewhere.
I am not amused. Now where did I leave my DriveImage disks...