All models of the Atari ST1 had code in ROM to attempt to read and run a boot sector from the first sector an ACSI2-attached drive. The bootloader3 contained in that sector would4 load the hard disk driver from a known file5 in the hard drive's root directory6. This program would install the full hard disk driver and boot would proceed normally using the hard drive's root directory,
AUTO folder, etc.
1 The very first version of the 520ST loaded TOS from floppy disk. I am unsure of how it behaved when presented with a hard-drive at boot time.
2 ACSI was very closely related to SCSI, see here for details. All of the hard drives I used on the Atari ST/TT were natively SCSI with a small adapter (ICD Link 2, about 2×4×6 cm IIRC) placed inline between the ST's ACSI port and the SCSI cable. Later models such as the TT used SCSI natively and booted directly from their internal hard drive; the TT provided both ACSI and SCSI-2 ports on its back panel. The Falcon was a bit different; it used an internal IDE bus (PIO, no DMA) and an external (DMA-capable) SCSI-2 port (no ACSI port was provided).
3 The bootloader would be installed onto the hard drive using a stand-alone utility, typically named
4 Based on a combination of my recollections and the info on this page. AFAICR I haven't used a hard drive with any 520ST model only the 1040STf (early, TOS 1.0), 1040STe (TOS 1.62 or 2.06, forget which), and TT030 (TOS 3.x) models.
5 Originally this was
SHDRIVER.SYS (for SH204, SH205, and Megafile) and I believe was specific to the drive's controller board. I used the third-party
ICDBOOT.SYS (for a 40MB drive) and
SUPBOOT.SYS (for a 720MB Supra drive) on my drives. I don't recall what the TT's internal drive used (the driver may have been built into the TT's version of TOS).
6 One could also install the hard disk driver by placing the driver (
AHDI.PRG) in the
AUTO folder of one's normal boot (floppy) disk. The
SHDRIVER.SYS file is simply a renamed copy of this program.