Error code 0x8021100F seems to be returned in some places by ata.prx. More specifically, it seems to occur if the value at the address 0xBD700007 isn't divisible by 2 (or at least if bit 0 is set) after calling sceAtaAccessDataPort()? Not sure.
Looking at the PSP memory map, it does seem to be in the ATA/UMDMAN hardware register region, at least.
Edit 1: This seems to have more info about those hardware registers.
Edit 2: Looks like those functions might be meant for the DVD drive present on devkits? ata.prx has two functions - sceAtaIsDvdDrive() and sceAtaIsUmdDrive() - both of which seem to read from the same location. The functions just read from 0x00000000, but I assume the system redirects these accordingly upon loading. (Or maybe it's intentionally disabled like this?)
IsDvdDrive returns whatever value is currently there, but IsUmdDrive returns 0 or 1 if the (unsigned) value there is less than 1 (in other words a bool on whether the value is 0 or not.) And both functions that later call sceUmdExecGetConfigurationCmd() and sceUmdExecReadDiscInfoCmd(), first also call sceAtaIsUmdDrive() and exit out of it, if it doesn't return 0.