yuo should not have to buy a new card. /sdcard/ on file explorer may not be the sd card. wouldnt hurt to pull the card out,put it in a card reader,and make sure that where the file is.
as was said,the rules for PB31IMG to flash are simple:
1)root of sd card
2)proper,zipped file(leave zipped,make sure md5 sum matches)
3)sd card formatted FAT32
4)proper name(phone must see PB31IMG.zip so name with windows to "PB31IMG" )
i know you have checked and double checked,but one of those things is not right if its not finding and checking the file.
an alternative since youre becoming fustrated might be to run an RUU on your PC,its alot more straightforward: download,run as administrator(if possible),then follow on screen prompts and plug in phone when directed.
download this RUU: http://pvillecomp.com/wp/wp-content/uploads/Images/RUU_Incredible_C_Froyo_VERIZON_WWE_3.26.605.1_Radi o_2.15.00.07.28_2k4k_NV_1.70_PRL58006_release_1520 16_signed.exe
wich was found on: Stock Images | PvilleComp's