Seagate External HD constantly needing to repair

[2056]

I have two seagate external hard drives. one is a 250 gb that works great..never had any problems. i soon ran out of room and had to purchase another hard drive which is the 500 gb freeagent desktop. since i've been using the hard drive.. whenever i want to access files i'll try to open it and it shows it's empty. so, i have to go into the disk utility to repair it and then the files are back.
it's starting to get annoying. i don't even have to turn the imac off for this to happen. hours later they will just disappear by themselves.
any suggestions?

[2056]

ahh, this is just getting so ridiculous now. hours of burning and saving files on the hard drive. some of them are there..some are not the next day. it's all very random and i'm not understanding it.

i'm hoping someone could offer advice.
i've tried to reformat (extended journaled) the drive to start off fresh..