"ERROR 7: Error on rename of..." when altering table

[Andreas Hammarstedt]

I was trying to alter (ALTER TABLE) a MyISAM table on our production system to add several columns. The table is 13 M and some records and about 2.5 GB in size. The platform is W2k3 with MySQL 4.0.20.
After well over an hour of processing, the alter query returns:

ERROR 7: Error on rename of '.\data\#sql-5b0_8ce.MYI' to '.\data\data_txt.MYI' (Errcode: 13)

I am not seeing any permission issues with the file or directory, and the query worked fine on our test system (albeit on a much smaller DB: <4 M records and 700 MB.)
It appears that the temp index and data files are complete (similar sizes to the originals) and that replacing the original files is what is failing.
I've looked through the docs and this forum and am not finding any hints. Does anyone have any ideas?

TIA.
-Andreas