For months X-Plane's web-based installer has been returning I/O errors randomly to users withithe new Intel Macs. The specific offending routine was FSWriteFork but we couldn't figure out why.
We eventually turned to DTS, Apple's third party developer support team - you can buy a tech support "incident" from them - that is, assistance with one bug. It was from them that I got the missing tidbit of information:
Usually ioErr (-36) means something has gone deeply wrong at the lowest levels. Here's what the actual header file says if you have Apple's Carbon SDK:
ioErr = -36, /*I/O error (bummers)*/
That is, generally speaking, application programs can't recover from this - it would usually mean the hard drive is going bad. By comparison paramErr (-50) means you passed something illegal to the function.
Well it turns out that if you pass a block of memory to FSWriteFork and part of that memory is not mapped (that is, trying to access that memory to do the disk write generates a segfault) then you get ioErr back, not paramErr. That tip led us to realize that what we really had was (blushes) bad endian-swapping code that (on little-endian Macs only) generated bogus pointers. Depending on luck and the stars, that bogus memory might be mapped (thus writing junk into the file) or unmapped (generating the -36 error), hence the unreliable behavior.
In hindsight this isn't surprising, once you realize that Mac Carbon file calls are written on top of BSD file manager calls. I don't know the actual implementation of FSWriteFork, but what if it uses BSD's "write"? Well, that can return EFAULT if you pass in unmapped memory.