your dump looks pretty peculiar, it seems to miss the first part, but does contain
(part of) a readable dump
Are you sure you copied empty.dum and diskboot.bin to an EMPTY bootable card ?
Udumper is really 'primitive', you see, it doesn't know anything about filesystems
(which is not surprising considering it's effectively only 175 bytes 'big' )
it just starts dumping from sector 2048 on the card; that's about 1 mb from the
beginning of the card. So if there's more than 1mb on the card when your copying
empty.dum on it it will overwrite (part of) already existing data ...
on the other hand, if your card WAS empty, then this udumper is no good ...