No it's not the 4 GB thing. I know that, and as was said it would tell me, the 4 GB thing doesn't mess up external HDD when you try, it's just the error message.

The problem is when just moving or copying things over, the HDD messes up and shows no files in my HDD's anymore. Only the two that i'm doing it two messes up. Sometimes only one will mess up and it's different sometimes. The file i'm moving from may mess up and show no files anymore, or the one i'm moving it too may mess up and show no files.

Another thing I just thought of is that it could be your PSU is unable to supply power to the external HDD anymore, although this does seem a bit odd and unlikely, my brother's PSU did start to conk out like this. If other components start to behave erratically in the future and the PC has a tough time turning on then that would point to the PSU. Unlikely but possible.

I still say you try to format a HDD into NTFS, although it would mean having to backup your data first.

No it's not the PSU. I think this is something nobody can find out. Because, it doesn't make sense. I have run some tools that come with my HP on my External HDD and it says they are all working fine. I have run a program that checks the devices. Sometimes it says a Disk Volume error is there that i need to reboot, i do and it doesn't say it but then it says something about the Hardware not being installed right. I have ejected and replaced my HDD many times and they never seem to work better.

I actually think there is something wrong with the USB ports. I moved each external to a different port but the same area, they are alowing me to transfer files over so far, but now one of them keeps messing up when i run a scanner on it, or when my computer updated it messed up. I just don't get it. Isn't USB 2.0 standard now?