In Ubuntu 10.10, we changed the normal
amd64 CD images to dual-boot on either BIOS or UEFI systems (UEFI, "Unified Extensible Firmware Interface", is a different kind of firmware found on many newer systems). This was done using a technique known as a "multi-catalog" CD - it contains two boot images, and the specification says that the firmware is supposed to pick the one it can best use.
Unfortunately, even though Macs use a variant of EFI (an earlier version of what's now called UEFI), they apparently can't cope with multi-catalog CDs, and simply refuse to boot them. This left us in rather a quandary: we needed to support UEFI systems, but we didn't want to drop support for Macs either. I therefore created the
amd64+mac CD images, which are exactly the same as the
amd64 images except that they only support BIOS booting. Macs are happy to boot these in their BIOS emulation mode.
(In fact, the name
amd64+mac is a slight misnomer, because it later turned out that some systems other than Macs suffer from a similar problem - but I felt that a more technically accurate naming such as
amd64+nouefi would be more likely to confuse than enlighten.)
While I would love to return to shipping just
amd64 images rather than both
amd64+mac, at the moment there is no prospect of reunifying them unless somebody figures out how to make a multi-catalog CD image that Macs can boot. If you're an expert on this, please do contact me by e-mail.