The one C:H:S address we can translate without knowing geometry is the
address C:H:S=0:0:1 which always means lba 0. I haven't yet heard of a
BIOS willing to translate that address while unwilling to translate
less unambiguous addresses.
Right, but *somehow* wrong .
... what we want to do is to find a start (BOOT) sector address for the FIRST partition that can be reached both with LBA and CHS and with ALL the geometries seen above, the only one that fulfills this is Cyl 0 Head 0, we could start as low as Sector 2 but as some utilities use the first few hidden sectors for their data, including the “normal” FAT32, best option is to get the last addressable sector with geometry 125*64*32, BEFORE “switching” to Cyl 0 Head 1, i.e. Cyl 0 Head 0 Sector 32, this way even if cylinders, heads and sectors number differs, the result is GUARANTEED, let us set the size of the partition , so that the entire partition can be accessed again BOTH as LBA and CHS on ALL geometries.
any address belonging to Cyl 0 Head 0 (with sector number smaller than geometry max sector) is actually translated correctly, and since number of sectors is anyway at least 32 in observed media (actually either 32 or 63) 0/0/32 is usually "good enough".
About USB ZIP drives, you are falling for a historical trap, you cannot even think about USB ZIP drives without knowing how/what their previous incarnations were.
The ZIP drive was born as an external device and came in two flavours, the SCSI and the parallel one, the parallel was never meant to be booted while the SCSI was AFAICR bootable fine on some "real" SCSI cards, not with the el-cheapo one that was given as option with SCSI disk drives, which had no BIOS Extender/was not bootable (and yes I did have a Windowsw NT 4.00 "emergency" system booting from a SCSI ZIP).
I lied , there WAS a way:
unfortunately the good guys at Blue Sky innovations prevented the Wayback machine from caching their pages a brief excerpt can be found here):
The "IDE insider" came third and as said was soon replaced by the ATAPI, ATAPI2 and ATAPI3 versions.
The USB was the last one and it was an adaptation to a brand new technology (the USB bus) of a good ol' one, the ZIP drive came out like in 1994 or so, by the time the USB version came it was already "established" technology and when USB came, it was slow, the "pros" had all the (fastish) SCSI version or the ATAPI internal one, and soon the 100 Mb (which had become later 250) was too small and the later 750 version was more like a meteor than anything else.
So, all in all there are (were) more ZIP drives models than stars in the sky, and it is not at all easy to know what/which did what.
All devices - including USB sticks - AFAIK do report their geometry, the point is that every BIOS around may choose to respect it, translate it to something else, expect a specific one and/or fake a completely different one.
It is (maybe was) a mess.