<Of course, I was just using the vernacular... (About like "CMOS setup" - <you didn't make any comments about that one though. :-)) But I didn't <remember that it was called ST506. Sounds like a Seagate part number. <Was it a PC hard drive, or even earlier? The ST506 was an early 5mb full height 5.25" hard disk from Shugart (later to become seagate). I still have a few. I predates the PC! The first PCs to have a hard disk used either the ST506 (5m) ST412 (10mb) or the Tandon t100 10mb all of which had a similar st506 drive level interface. <> You're probably thinking of the servo bursts - the signals that keep th <> heads on a track. Some drives did use a particular side of one of the <> platters for these. Other drives 'embedded' them in the sector headers <> the data platters. The latter is not common on ST506 interfaced drives, <> but is common on SCSI/IDE drives < <Hmmm, interesting. So low-level formatting doesn't rewrite these? <That would imply that the servo bursts are not involved in determining <sectors at all. I used to use an RLL card with a couple of formerly <MFM drives, and it made more sectors per track, so I thought the <wasted platter had something to do with that. Servo bursts are written at the factory and if lost the drive is junk as there is no way to reconstitue them. Their primary function is to get the head correctly centered on the cylinder regardless of temperature, vibrations and mechanical wear. MFM vs RLL... The media has a capacity to store a certain number of magnetic transistions per linear inch. This is a design number and unchangeable. What can change is the interpretation of those. RLL uses fewer magnetic transistions to encode data and it's clocking information needed to recover the data later. RLL in effect is data compression. <> Are you sure: While almost all clone controllers have a formatter routi <> at C800:5, < <Yep, that's the one. < <> I couldn't find it in the original IBM XT hard disk BIOS. Some of the controllers didn't have rom and depended on a floppy loaded formatter or floppy loaded driver routine. On one of my floppies I have HDINIT that was used for that purpose. Allison