GreatLord wrote:alexei it is not so simple as you think
we need...
No, we don't
To be precise, we can live without it for a while
If you ever track NT loading from USB (or HDD) you would know that it uses Int 13h to access the drive from the very beginning (MBR code) to the point where all boot-time drivers already loaded and started. Also, you probably know that there are a lot of drivers that substitute HDD with some kind of "emulator". To name a few: RamDisk, EWF, SDI, Filedisk.
Modern BIOSes already have USB stack implemented. It allows them to provide unified access any HDD (PATA/SATA/SCSI/USB) via Int 13h calls.
When you select booting from USB-HDD in BIOS setup, the USB drive becomes accessible as drive 80h, which by Int 13h specification is supposed to be 1st HDD. The driver can call int 13h instead of "talking to" HDD controller, though 16-bit thunk is obviously needed to call BIOS interrupts.
As a future perspective it would be better if ReactOS had unified HDD driver that would initially use BIOS (or EFI) and then switch to using appropriate driver for the specific device PATA/SATA/SCSI/USB/Firewire. That would also allow not to use "special" NTLDR (EWFLDR) to support EWF, etc. Unified HDD driver would also superseed NTBOOTDD functionality.
And, as a final note, "Portable NT" is one of the MS worse nightmares, so they made their solutions accordingly
Sorry, if my initial post sounded incompetent to you.
Alexei