There are several layers at which a disk error can be simulated. If you are testing a single user-space program, probably the simplest approach is to interpose the appropriate calls (e.g. write()) and have them sometimes return an error. The libfiu fault-injection library can do this using its fiu-run tool.
Another approach is to use a kernel driver that can pass through data to/from another device, but inject faults along the way. You can then mount the device and use it from any application as if it was a faulty disk. The fsdisk driver is an example of this.
There is also a fault injection infrastructure that has been merged in to the Linux kernel, although you will probably need to reconfigure your kernel to enable it. It is documented in Documentation/fault-injection/fault-injection.txt. This is useful for testing kernel code.
It is also possible to use SystemTap to inject faults at the kernel level. See The SCSI fault injection test and Kernel Fault injection using SystemTap.