The Free On-line Dictionary of Computing (30 December 2018):

Commodore 1581
1581

    Commodore Business Machines's 3.5 inch disk
   drive for the Commodore 64 and Commodore 128.  The drive
   stores 800 kilobytes using an MFM format which is
   different from both messy-dos 720 kb, and the Amiga 880 kb
   formats.

   The 1581 supports a poor imitation of directories which are
   really just partitions and largely unused.  It also supports
   burst loading like the Commodore 1571, but is actually
   faster as it is better designed.  It has 3160 blocks free
   when formatted.

   The 1581 is the highest density C64 serial bus drive made by
   Commodore.  However Creative Micro Designs (CMD) make the
   FD2000 (1.6MB) and (until recently) the FD4000 (3.2MB)
   3.5" disk drives.  GEOS users like 1581s as they are very
   fast when used with GEOS.

   See also Commodore 1541, Commodore 1571.

   (1998-12-23)