This is 90% likely your issue.

KV is from a newer console (usually slim but could still be a phat) and has FCRT.bin required flag on it. Some consoles (older ones I think), don't use FCRT.bin in the NAND. If I remember correctly, FRCT is associated with drives and some drives don't use it.

If the the kv has a flag for it being required and your console does not use it, then it won't boot.
To fix this do the following:

1. Load NAND in 360 flash tool, import kv and change drive info, DVD key, etc
2. Load the NAND with new kv into latest Xebuild, and input CPUkey
3. Click check kv in bottom right. FCRT bin required will probably be true
4. Under special patches, tick nofcrt patch
5. Click generate hacked image and flash this to console


As for people saying to put on hdd, please don't bother posting. He obviously wants to flash to his console. I flash kvs to the console, because a) some kvs don't work on the root of the hdd for some reason, b)you are a lot safer against kv stealers.

EDIT:
Just read your post again and seen this:

***** ERROR: fcrt.bin not found, keyvault has this file flagged as required but it's being skipped *****
note - on hacked images a nofcrt patch WILL NOT allow the drive to function
but should allow the machine to boot normally


Don't why it says the console will boot, because mine doesn't with kvs with a flag for fcrt. Also my drive works fine, so don't know why it says that either.

yes i dont know why its the weirdest experience i have had with kv's

Lul a slim is a slim no matter what even if you convert it. It wint read it. Buy a phat kv preferably one pulled out of a jasper