The story
The 1571 was the natural companion to the Commodore 128. Announced with that computer in the summer of 1985 and widely available by the end of the year at about US$300, it brought two read/write heads to the Commodore line, so it could use both sides of a disk at once. A double-sided disk formatted on a 1571 holds 340 KB, twice the capacity of a 1541.
The drive runs a 6502 at 2 MHz alongside a Western Digital WD1770 floppy controller, with 32 KB of ROM containing CBM DOS 3.0. Its biggest advance was burst mode, which used a proper hardware shift register to move data to the C128 far more quickly than the 1541's software-driven transfers. The controller also let it handle MFM disks as well as Commodore's own GCR format, opening the door to many CP/M disk formats from other machines, and with third-party software such as Big Blue Reader it could even exchange files with MS-DOS disks. Track-zero sensing ended the 1541's head-knocking problem, and the drive ran cooler and quieter.
Compatibility needed some care. Attached to a C64, VIC-20, Plus/4 or PET, the drive starts in single-sided mode, and a C128 started straight into 64 mode does the same, so double-sided disks cannot be read until the mode is switched. Disks written double-sided cannot be read on a 1541, and some copy-protected games that worked the 1541 hardware directly failed on the 1571. Early ROMs could also corrupt relative files spanning both sides of a disk; a later revision fixed this but introduced quirks of its own.
The European plastic C128D had a 1571 built in, while the later metal-cased C128DCR used a revised DOS 3.1 and a simplified support chip. A dual-drive 1572 was shown at the 1985 Consumer Electronics Show but was cancelled, reportedly because of DOS problems. Demand at first outstripped supply, and briefly tempted two clone makers until Commodore's lawyers stepped in. The 1571 stayed in production until 1993 and remains the drive of choice for many C128 enthusiasts.