> i never run a "quiet" kernel parameter because *normally* i find the boot up process very edifying. but in this case -all- i get for error messages is (sorry for repeating myself) "Gave up waiting for root device ...<correct UUID> ...does not exist" then busybox. no hint as to why this kernel compile can't find the root device when all those previous to 3.10.0-rc1 can.
It's hard to debug without more information (even non-error output). Without anything more specific, some other suggestions would be: From busybox, check dmesg to see if/when the HDD showed up, check whether the device actually exists in /dev, check whether LVM or RAID volumes were set up (if you're using them). Compare the complete exact boot output from the working kernel and the nonworking, see where they differ (either with netconsole, or just a digital camera). Extract the initrds and see if they differ.
> no hint as to why this kernel compile can't find the root device when all those previous to 3.10.0-rc1 can.
It could be something as hidden as a timing difference in when the disks get detected, for example. If it's really a kernel issue then a bisect is the more surefire way to find it.