Something seriously weird is happening...

Okay, after several minutes of trying to make this much harder, I
discovered that swaana.00008.hdr is corrupted. The rest of them
seem to be fine.

to3d -epan -time:zt 69 209 2000 alt+z swaana.*.hdr

that line should work, once you delete swaana.00008.hdr and .img.

Or you can just overwrite the bad hdr with a copy of the next one:

cp swaana.00009.hdr swaana.00008.hdr

and then you can use the command:

to3d -epan -time:zt 69 210 2000 alt+z swaana.*.hdr

Or you can figure out what happened to the swaana.00008.hdr,
fix it, and use that same command. But I think copying over it
is the easier answer, and seems to work just fine.

rich