I have almost the same issue with my DELL Latitude E6530.
The BIOS (with the latest update) does not recognize the BX100 SSD.
Via DISPART I was able to format the SSD so now windows (7/10) does recognize the SSDvia USB mount but still the BIOS doesn't. Pls Help
It looks like the newer ThinkPad T420 BIOS prevents use of the BX100 SSD, I guess it may be possible to find an older version of the firware and downgrade it to allow compatibility.