Unfortunately, with AIX 5.2 (forgot which ML, but quite old like yours) and nim alt_disk_install - I got the same failure - and while suggesting it as an improvement to the standard using CDs method during a class.
I sure felt foolish then. And I have been afraid to try it since.

Maybe updating to a newer version of AIX 5.2 will help - but it is probably a lot easier to just do the migration using CD's rather than nim. However, if you have a lot of systems to migrate I would suggest installing a test system with AIX 5.2, update that the latest AIX 5.2 ML (10 I believe) and try the nim based alt install and see if that works. If it does it may be worthwhile to do a update_all to your AIX 5.2 systems, and then perform the migrations.

btw: there are some tricky issues with updates toAIX 5.3 ML6 and ML7 (I recommend updating to ML7 since you are doing all the work anyway). Make sure your /tmp has at least 90M of free space so that the bosboot can be built.