Bash – replace a ‘space’ char in filename with an underscore

Put quotes around $x everywhere it appears.

for x in `find . -regex '.*/\*.*'` ; do mv "$x" `echo "$x" | sed s/\*/_/g` ; done

Try mounting the filesystem with the iocharset option set to the encoding it uses.

From man mount under the "Mount options for fat" section:

   iocharset=value
          Character set to use for converting between 8 bit characters and
          16 bit Unicode characters. The default is iso8859-1.  Long file‐
          names are stored on disk in Unicode format.

See also under the "Mount options for vfat" section:

   uni_xlate
          Translate  unhandled  Unicode  characters  to  special   escaped
          sequences.   This lets you backup and restore filenames that are
          created with any Unicode characters. Without this option, a  '?'
          is used when no translation is possible. The escape character is
          ':' because it is otherwise illegal on the vfat filesystem.  The
          escape  sequence  that gets used, where u is the unicode charac‐
          ter, is: ':', (u & 0x3f), ((u>>6) & 0x3f), (u>>12).

and

   utf8   UTF8  is  the  filesystem safe 8-bit encoding of Unicode that is
          used by the console. It can be be  enabled  for  the  filesystem
          with this option or disabled with utf8=0, utf8=no or utf8=false.
          If `uni_xlate' gets set, UTF8 gets disabled.

Edit:

I'm sorry, that was Linux, this is for BSD (from man mount_msdosfs:

 -L locale
     Specify locale name used for file name conversions for DOS and
     Win'95 names.  By default ISO 8859-1 assumed as local character
     set.

 -D DOS_codepage
     Specify the MS-DOS code page (aka IBM/OEM code page) name used
     for file name conversions for DOS names.