Normally WimBoot only supports optimized WIM files, exported or captured with the /WimBoot parameter. These files have XPRESS compression with smaller dictionary size. However, normal XPRESS or LZX compressed WIM files can be used, too. But not esd files. Not sure what's the exact requirement for WIM file placement, definitive not CDFS/UDF, maybe not even removable drives at all. But for sure local NTFS drives.