I have original mbox on a vaio fit15 i7 laptop running PT 8.0.1 just fine under windows 7. -And, now runs just fine after upgrading to win 10. Doesn't even need compatibility mode. Seems to work good even with both system and audio on the same c: evo 850 ssd drive. Not having problems writing to the system drive seems to strictly be a matter of whether your hardware can keep up the demand for speed or not. Pro tools doesn't always fail gracefully before version 9 or 10.
This system build did however, require mbr (master boot record) partition table format and ntfs basic disks. Set bios to legacy boot, repartition and install fresh copy of windows to insure you get mbr not gpt. This also implies clone images need to be made from an mbr partition disk or you may need to do fresh installs. But, only if you want or need the portability of recording to the system drive, or lack enough usb ports for midi, external drive, and interface. My vaio halves the usb 3 speed if both ports are used at the same time.
However, I also noticed it seemed to use a good bit more cpu and memory when the system drive was on gpt, even though it was writing to an external mbr formatted ssd usb 3 disk. Cpu use was 50% on all 4 cores, but is now 10% or less. Not sure why unless its just that much more work to write to extrrnal usb. -which would contradict a lot of advice.
PTLE 8 could not see the c: drive as an audio volume if it was using uefi global partition table (gpt). Took many hours to figure that one out. I don't think macs like gpt, either, even though it is formatted hfS+. -At least not with version 8, anyway. Also, be sure to right click the pt icon and run as administrator on win10 even if you have an administrator account.
This has breathed new life into an old origional mbox system. The boat anchor not only lives, it now travels.
Not supported is not the same as won't work. It just may not be for the faint of heart. RAID 0 has never been supported either, but has always worked fine for me. Avid just doesn't want to do the legwork to discover gotchas like the gpt partitions and waits for their users to do it for them. I guess they dont even want to add the stuff users know works, because they would have to keep supporting it forever.