Are there any byte-level RAID standards, and if not why?
Disks are block level devices, meaning you can't read and write single bytes from them, but only entire blocks. Traditionally this has been 512 bytes, but 4096 bytes is becoming common.
There is almost always overhead in communicating to a hardware device - if the amount of data you request is very small, you will spend more time in the overhead than getting the data. As disks are a mass storage medium, meant to store mass amounts of data and not single bytes, working on the byte level is probably not worth it most of the time.
As far as the sweet spot - 512 has been in use for a long time. 4096 is around only because hard drives are requiring higher physical density to store more data. With old tape drives that could work like block devices I believe you could change the block size. IBM's original block size of 128 was chosen because it was the next largest power of 2 above 80 - and 80 is the number of characters on a standard typewritten line.
Are there any standards for how data is stored on the physical disks
For hardware RAID, there is probably not unless a vendor has published this information somewhere and they probably don't want to make it public for reasons below.
For software RAID: I am unaware of any source of information on the format of Windows dynamic volumes (which let you do software RAID) but it may be out there in some Microsoft document. The format Linux uses for lvm
/md
disks is at least documented in the source code and probably explained in other easy-to-Google places.
and if that's not the case, then why not?
The pessimistic answer is that vendors are incentivized to lock you in - if you have a RAID controller that goes belly up and need a replacement ASAP because all your disks are formatted to work with that controller, you're going to buy from the vendor again.
The optimistic answer is that this allows a hardware vendor to optimize the structure on the disk for the hardware and/or a software/firmware designer to optimize the structure on the disk for the algorithms or techniques involved.
Barring a "universal" standard, what makes RAID implementations interoperable?
An agreement on what blocks/areas on the physical disk mean what.