To quote the Wikipedia article on RAID:
A RAID 0 can be created with disks of differing sizes, but the storage space added to the array by each disk is limited to the size of the smallest disk. For example, if a 100 GB disk is striped together with a 350 GB disk, the size of the array will be 200 GB (100 GB × 2).
The article doesn't, however, state why it can't RAID them together and use up the entire disk space. I understand that the striping writes between the two, but surely it shouldn't prevent writing to the later sectors of the second disk, whilst running in degraded performance? Is there an alternative RAID system (e.g. software RAID) that offers such a feature?