Data recovery from CRC error on QIC cartridges…

Quarter Inch Cartridges or QIC for short is a magnetic tape introduced by IBM I 1972 and was widely used up until the 1990s. Data can be written linearly on a single track along the length of the tape or it can be serialised and written ‘serpentine’ one track at a time with each track written in the opposite direction to the adjacent track. It contains and endless drive belt which is moved uniformly by a motorised capstan wheel. Our client had a DC6150 18 track cartridge which they were unable to read.

The drive belt had lost elasticity and frayed. It was replaced before the recovery analysis could be continued. This is an extremely fiddly job and takes the best part of a day to do. Once the work was completed, the news wasn’t great!

We managed to rebuild the tape and read off the first four files before we hit the first bad block (a hard CRC error, which the drive refuses to read past). Due to a mix of frayed drive belt, and the client repeatedly trying to read this tape and not getting any further than the first 300Kb, the tape had become grossly mal-distributed across the spool. This in turn led to heating and actually distorted the tape itself.

We had to rebuild the cartridge repeatedly in order to seek the EOD marker. These were attempts to fool the drive into going beyond the bad block and rewinding to a point just past it. However the tape rapidly destabilised, wobbled and then threw the belt off the spool.

These QIC cartridges were originally manufactured by machine using high precision jigs and once they lose tensional "symmetry" on one or other spool it's sometimes impossible to rebuild them to the point they're stable enough that a drive will read them.

However, the client’s associates had another boot tape that could be used. Thankfully we were able to instruct them how to duplicate it on-site - a process that no more endangers the tape than booting off it.