what is the difference between softerrors,harderrors,transport errors?
this is not quite true but a good guide line:
soft errors are like CRC corrected errors.
hard errors can't be corrected and are very bad.
transport errors are a communication issue on the SCSI bus.
the technically correct version is very hard to understand and linked in the other reply 
Not that "device not ready" is listed as a hard error but is usually harmless when appearing on a removable media like a DVD reader.