300 GB is a bit big for FAT32. It if really is pre-formatted for the PC (manufacturers mean WINDOWS when they say PC), it's probably NTFS which isn't supported by the PS3. If it IS FAT32, it may be too big for the driver in the PS3 (many folks assume limits on types of drives and filesystems that eventually get exceeded... for example, Microsoft couldn't even use FAT32 on drives larger than 30-some gigs for quite a few years).
So check the drive - if it's NTFS, reformat it as FAT32. If it is FAT32, repartition the drive to chop it into smaller pieces, format the partitions as FAT32, then try it again.