Rac,
Sorry, I mis-spoke. The phenomenon is “hard drive wrap-around” which Wired_w, addressed wonderfully on the “Maximum total drive storage allowed on My Cloud (v2)?” page. (His excellent explanation of that can be found here.)
In short, the phenomenon of hard drive wrap-around occurs when either the hardware or the operating system do not have enough address bits to access the entire drive.
In Windows XP, there was an operating system limitation of 137 gigs, even though, (theoretically), the NTFS file system could support drives in the Petabyte range. The problem was that hard drives had grown to the point where 500 Gig+ drives were available, people were plopping them into systems - as I had done - and discovered that the “Petabytes of storage” hard drive limit was an illusion.
What would happen is that the address for the “next” block of data would “overflow” the address bits maintained by Windows, causing it to wrap around to sector zero and continue from there. Result: An unrecoverable hard drive.
Modern operating systems - post XP - are designed to avoid this problem by using a larger addressing scheme with 48 bits of address space instead of 32, which adds a huge amount of space.
What complicates matters even further is that the new SATA/eSATA interface spec doesn’t care what size the drive is or how many bits are used. All it expects is that the devices at each end know what they’re talking about.
Ultimately it becomes a matter of what the hardware controllers can address, (as bits are not an unlimited resource), or what the operating system can handle given the limitations of whatever modifications were made to it or it’s available memory.
Using Wired_w’s Windows XP example from his excellent explanation within that article, Windows would wrap at 137 gigs because of the LBA-32 addressing scheme, which LBA-48 solved.
When discussing embedded hardware, simply because the spec specifies 48 available addressing bits, doesn’t mean that all of them will be implemented in expensive silicon real-estate. This is especially true with embedded hardware where both cost, and physical size, are constraints that limit what the chip designer can do.
If there are fewer bits in the hardware registers, or if there is a lack of available controller/system memory, the actual limitation of the system can fall far short of the Petabytes and Exabytes that are theoretically possible.
The result has the potential for ranging from a crashed system, and potential drive corruption, or an address wrap-around in a controller chip somewhere, causing all the data on the hard drive to be lost.
Since I do not know which - if any - of those possibilities are true for the My Cloud, I have to assume that there is some limit, as yet unknown. And as I said on the other thread, we will probably not find out until someone adds more space to his device than it can handle and he borks his box. Which will be an absolute shame.
What say ye?
Jim “JR”
