No, subway magnetic fields might not cause hard drive data corruption and erasure based on the measurements taken for magnetic field strengths on the electrically powered trains which operate on alternating current such as the New York city subway and the Baltimore Washington commuter train. The measurements taken on the Baltimore Washington commuter train in 1991 showed 25 hertz magnetic field strengths as high as 500 milligauss in passenger areas at seat height. This is calculated as 500 milligauss equals to 0.5 Gauss (0.5 Oe) which is very minimal compared to the required magnetic field strength of 5000 gauss and above needed to erase data from hard drives.
The highest magnetic field '30 milligauss' measured in Toronto subway is actually 0.03 Gauss (0.03 Oe) which is also below the magnetic fields needed to erase data from hard drives. Magda Havas et. al. 2004 reported that,
The highest magnetic fields were found in subways (mean 30 mG, range 3 to 100 mG), followed by streetcars (mean 30 mG, range 2 to 100 mG), buses (mean 11 mG, range 1 to 50 mG) and the GO-train (mean 2 mG, range 1.2 to 2.8 mG). The magnetic field increased with acceleration and deceleration and varied with seat location and this was most obvious in subways and streetcars.
After the data is stored, a strong interfering magnetic field could reverse the orientation of some or all of the grains in a bit cell, possibly converting a zero to a one or vice versa causing corruption of the data in the hard disk. The ability of the magnetic material to resist this phenomenon is known as coercivity and modern magnetic media with Cobalt based coatings in disk drives can be on the order of 1000 times higher coercivity than earlier used storage media Yang, 1991.
"DC magnetic field exposure at and above 255 Gauss causes undesired but temporary performance degradation in the hard drives tested. Exposures up to 305 Gauss caused no loss of data, and no permanent damage".
Two different models of hard drives were tested in non-time varying magnetic fields of various strengths. The impact of the magnetic flux exposure was quantified by performing data transfer rate benchmark testing, data integrity tests, predictive testing, and surface scanning on the drives before, during and after exposure. The drives were found to be largely immune to magnetic flux densities up to approximately 250 Gauss (0.025 Tesla).
The earliest hard disks (IBM’s 1956 model) used iron oxide as the magnetic material. The low coercivity of iron compounds explains that there was little resistance to data corruption from external magnetic fields. Iron compounds were commonly used in the floppy disks of the 1980’s and 1990’s. Floppy disks are vulnerable to damage from even relatively weak magnets Keizer, 2004.
Small rare earth magnets are commonly used in consumer products, and their usage is increasing. For example, the iPad 2 product by Apple Inc contains 31 magnets, 21 in the device’s folding cover, and another 10 in the iPad itself. Yet the servers, laptops and hard drives in proximity to all these iPad magnets are not failing.
The only magnets powerful enough to remove all of the data from a magnetic hard drive are laboratory products known as electromagnetic media degaussers. These devices operate above 5000 Oe.