This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
Try using a slower mode or disable fast modes altogether. Mode 3 and especially mode 4 are very sensitive to timing problems, and not all adapters follow the ATA-2 specification really closely. Don't dismiss the possibility too easily: if you changed anything on your system, it is very well possible that a drive which marginally worked so far now starts to corrupt data.
Some controllers seem to configure themselves according to the capabilities of the master drive. This can mean trouble if the slave handles only slower modes.
Moreover, check your cables, and ensure they aren't too long (see Q5.4). Removable drive brackets may also cause problems with fast PIO modes for roughly the same reasons.