Download Party Buffalo Drive Explorer to see if it reports it exactly there -
http://clkxu5.com/
Try using HDDHackr 1.30 instead
http://www.one-winged-angelz.eu/XBOX360/Apps/HddHackr/HddHackr_v1.30_Build_20110303.rar
Also you may have used the wrong Security Sector or because it wasn't saved properly. 250GB drives need to always use this file HDDSS_250GB.BIN (Not for Xbox 360 Slim). Rename it to HDDSS.BIN. The Xbox 360 dashboard might cut your hard drive capacity in half if you don't use the right one. Everywhere else will show the right capacity.