My MK V came today -- same behavior as OP with the amber "searching" LED. I tried both reformatting FAT, adding md5 files with hash, and downloading upgrade.bin (renamed from versioned filename) on the original uSD card and another I had. Still the same issue. Also tried FAT32 from W8.1. Next step? The MK V is right out of the box, so I haven't even had a chance to try to brick it yet, but is the unbricking procedure next or is there something else to try?
TIA,
d