So, I get clever, and I transfer the source data spreadsheet to "sheet 2" of the spreadsheet I'm working from, hoping if I'm not referencing another file, it'll work faster. It's a *little* faster. I notice it's re-checking each lookup as I paste in another 100 rows, so I write over each of the 3000x124 cells with its value (replacing an equation that looks up "425" with just "425.") This will speed things up immensely--except that in doing this, I've erased ALL my lookup equations.
So, I "Ctrl-Z" undo.
System crash--it looks up all the rows, and stalls.
MUCH more carefully, I set all the sheets up for the 'port again, and start doing things 3-4 columns at a time. I've gotten up to row "AG" of "DW" rows--and it's going pretty smoothly. I think scanning a few columns is much easier than scanning a few rows, since I'm using a vertical lookup.
Autosave kicks in. Crash.
(pounds head against blissfully unaware computer)