Throughput and Software
Initially, Commodore intended to use a hardware shift register (one component of the 6522 VIA) to maintain relatively brisk drive speeds with the new serial interface. However, a hardware bug with this chip prevented the initial design from working as anticipated, and the ROM code was hastily rewritten to handle the entire operation in software. According to Jim Butterfield, this caused a speed reduction by a factor of five.
As implemented on the VIC-20 and Commodore 64, CBM DOS could transfer only about 300 bytes per second - compare the 300 baud data rate of the Commodore cassette storage system - which translated to about 20 minutes to copy one disk—10 minutes of reading time, and 10 minutes of writing time. However, since both the computer and the drive could easily be reprogrammed, third parties quickly wrote more efficient firmware that would speed up drive operations drastically. Without hardware modifications, some "fast loader" utilities managed to achieve speeds of up to 4 kB/s. The most common of these products were the Epyx FastLoad, the Final Cartridge, and the Action Replay plug-in ROM cartridges, which all had machine code monitor and disk editor software on board as well. The popular Commodore computer magazines of the era also entered the arena with type-in fast-load utilities, with Compute!'s Gazette publishing TurboDisk in 1985 and RUN publishing Sizzle in 1987.
Even though each 1541 had its own on board disk controller and disk operating system, it was not possible for a user to command two 1541 drives to copy a disk (one drive reading and the other writing) as with older dual drives like the 4040 and 8050 that were often found with the PET computer, and which the 1541 was backward compatible to (it could read 4040 disks but not write to them since its internal Operating System was essentially the same). Unfortunately, however, the routines in the 1541 disk operating system to enable disk copy were removed as it was intended to be a stand-alone unit. To copy from drive to drive, software running on the C64 was needed and it would first read from one drive into memory, then write out to the other. Only later when first Fast Hack'em then other disk backup programs were released was true drive to drive copying possible. One could then unplug the C64 itself from the drives (i.e. from the first drive in the daisy chain) and do something else with the computer as the drives proceeded to copy the entire disk. This is not a recommended practice as disconnecting the serial lead from a powered drive and/or computer could result in destruction of one or both of the port chips in the disk drive.
Read more about this topic: Commodore 1541