Try giving IUSR_ALPHA (which usually has a friendly name of “Internet Guest Account” or something “Full Control” NTFS permissions.
IIRC the “Read/Write/Execute” is just IIS perms. You need NTFS which has perms like Read, Write, Delete, Modify…and you might be getting caught up on Modify.
Browse to the directory using Explorer (not IIS) and right-click on the DB or the DB’s folder, choose the “Security” tab and you’ll see the perms there.