Sinclair decided to integrate the Microdrives into their new QL, as they were inexpensive compared to other emerging storage systems of the time. When formatted with a Sinclair QL each Microdrive cartridge usually yields around 110k to 120k of space for data, but this figure varies, dependant on the exact length of the loop of tape contained within the cartridge. The tape is divided into sectors of 512 bytes. Cartridges formatted with a Sinclair QL are not compatible with the ZX Spectrum (and vice versa) but can be re-formatted by whichever of the two computers that they are intended to be used with. The only other computer that was equipped with integrated Sinclair Microdrives was the ICL One Per Desk (AKA BT Merlin Tonto or Telecom Australia Computerphone) - cartridges formatted with the ICL are not natively compatible with the QL or ZX Spectrum.
Generally, Microdrive cartridges have a poor reputation due to early reliability issues, and even the official Sinclair literature states that they would not last forever and encourages regular back-ups of data onto other Microdrive cartridges. Software houses that sold software on Microdrive cartridges often guaranteed the content for no longer than a period of three months and the emphasis was always on making back-ups of important software and using those back-ups day-to-day instead of the original cartridges. Generally, with reliable Microdrive units to read and write to them they were usually not as unreliable as they are made out to be.
After the launch of third-party disk drive interfaces, a lot of QL users switched over to floppy disks for their preferred storage medium as they tended to be more reliable and could hold a lot more data. Other more advanced solutions came later, including improved QL compatible computers, none of which supported the use of Microdrive cartridges and after the passage of more than 35 years the humble Microdrive cartridge is very much an obsolete relic of the early QL days!
Despite their obsolescence, quite a large amount of original QL software was released on Microdrive cartridge and there are collectors who like to own original QL Microdrive software – even better if it still works! It is however unwise to attempt to use an old Microdrive cartridge straight out of the box, as it were, as the original pieces of sponge that press the tape against the drive head tend to disintegrate (or if they haven’t, they are almost guaranteed to do so after light use) and if small pieces of the sponge get into the cartridge they can contaminate the tape, compromise the data or in worse cases, jam the mechanism completely, rendering the cartridge completely useless. The solution to this problem is a procedure that has become known as ‘re-felting’. Re-felting a Microdrive cartridge involves removing the fragile original sponge piece completely, cutting a small piece of appropriate felt down to the correct size (about 4 x 2.5 x 1mm) and carefully sticking that onto the space that the original sponge piece once occupied. If done properly, this will allow most or sometimes even all of the content to be recovered – but by no means does it guarantee a fully data-intact cartridge, as bad sectors often develop over time, resulting in data corruption.