Just re-tried - I actually still had the mymod\data\animations\pack.idx and skeletons.idx deleted from last time I tried.

So, I tried deleting the \data\animations\pack.idx and skeletons.idx (i.e. the original unpacked files) too.

Result: black screen, "beep" error. ESC, back to desktop. Check log:

03:27:59.437 [system.io] [info] exists: found redmark/data/descr_skeleton.txt (from: F:\Medieval II Total War)
03:27:59.453 [system.io] [trace] file open,,redmark/data/descr_skeleton.txt,580330
03:27:59.453 [system.io] [info] open: found redmark/data/descr_skeleton.txt (from: F:\Medieval II Total War)
03:27:59.484 [system.io] [info] exists: found redmark/data/animations/pack.idx (from: F:\Medieval II Total War)
03:27:59.484 [system.io] [info] exists: found redmark/data/animations/pack.dat (from: F:\Medieval II Total War)
03:27:59.484 [system.io] [info] exists: missing redmark/data/animations/skeletons.idx
03:27:59.593 [system.io] [info] exists: found redmark/data/descr_skeleton.txt (from: F:\Medieval II Total War)
03:27:59.593 [system.io] [trace] file open,,redmark/data/descr_skeleton.txt,580330
03:27:59.593 [system.io] [info] open: found redmark/data/descr_skeleton.txt (from: F:\Medieval II Total War)
03:27:59.593 [system.io] [info] exists: missing redmark/data/animations/test_weapon/sword_1_primary_0.CAS
03:27:59.609 [system.io] [trace] file open,,data/animations/test_weapon/sword_1_primary_0.CAS,,not found
03:27:59.609 [system.io] [trace] pack open,packs/data_0.pack,data/animations/test_weapon/sword_1_primary_0.CAS,,not found
03:27:59.609 [system.io] [warning] open: data/animations/test_weapon/sword_1_primary_0.CAS is missing
03:27:59.609 [script.err] [error] Script Error in redmark/data/descr_skeleton.txt, at line 61, column 115
Failed to open cas file 'data/animations/test_weapon/sword_1_primary_0.CAS',
03:27:59.609 [data.invalid] [fatal] Animation database failed to load!
03:27:59.750 [system.rpt] [error] Medieval 2: Total War encountered an unspecified error and will now exit.


Note, the descr_skeleton.txt in this example is an umodded, original unpacked file. Line 61 is the first animation in the file. It doesn't rebuild the .data and .idx - the game won't load without those files.