Fix Ps3 Slim Won’t Recognize Wd Usb 3.0 External HDD
Fix Ps3 Slim does not Recognise Seagate Usb 3.0 External HDD

It is simply because the drive is not set up properly, include delete (forced) EFI partition if there is any, change from GPT to MBR and format it properly to FAT32 via guiformat.exe if you want the drive to be larger than 32Gb