It's probably the hard drive. I had a bad hard drive (it had suffered a head crash) which wouldn't boot. I was then trying to boot from a floppy to try and recover the drive. Every time, it would get halfway through, then soon after the "Welcome to Macintosh" box, it would try and read something off the hard drive, cause said drive to move the heads to the damaged region, drive would reload the driver, and the system would freeze. So just because you're booting from a floppy doesn't mean that the hard drive is ignored.

Alternatively, it could be loading the Finder (do any INITs load before the hang?), which in turn fails to mount the drive, causing a hang.

In any case it's most likely a bad drive, particularly if the good SE/30 failed with the suspected hard drive.