Kimdino wrote: cgdisk's presenting existing ext4 partitions (subsequently verified as such with 'GParted') as 'Microsoft basic data' does strike me as a bug,
fdisk, gdisk, cfdisk, and cgdisk are disk partitioning tools, they do not format file systems not do they even report (as far as I know) the file system of any formatted partitions. parted and gparted can do both partitioning and file system formatting. As Didier pointed out, a partition type doesn't necessary mean that it contains a file system that would seem to be consistent with the types name. In other words it is possible to have a partition with a partition type as Microsoft basic date formatted as ext4, as apparently your three partitions ( sd*1, sd*3, and sd*4) were. So it is not a bug in cgdisk, since cgdisk doesn't even care about what file system the partition have on them.
What you apparently had was three partitions all formatted with ext4 file system that were somehow identified as Microsoft Basic Data (0700) partition type. The Slackware installer when presenting candidate partitions for the / file system looks at the partition types of all available partitions on the designated disks and present only those with Linux partition types . Since none of your partitions where not designated as Linux types, the installed determined that there where no valid Linux disks. An easy way to fix this would have been to fire up cgdisk or parted and change the partition type to Linux.