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.