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 fault-injection library using its 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 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 to inject faults at the kernel level. See and .
A simple way to make a SCSI disk disappear with a 2.6 kernel is:
echo 1 > /sys/bus/scsi/devices/H:B:T:L/delete
(H:B:T:L is host, bus, target, LUN). To simulate the read-only case you'll have to use the fault injection methods that mark4o mentioned, though.
To add to mark4o's answer, you can also use Linux's Device Mapper to generate failing devices.
Device Mapper's delay device can be used to send read and write I/O of the same block to different underlying devices (it can also delay that I/O as its name suggests). Device Mapper's error device can be used to generate permanent errors when a particular block is accessed. By combining the two you can create a device where writes always fail but reads always succeed for a given area.
The above is a more complicated example of what is described in the question Simulate a faulty block device with read errors? (see for a simple Device Mapper example).
There is also a on the Unix & Linux question.