When you ran Disk Utility, what did it say? No errors detected?
Here's what is going on: If you see the drive mount, but files are missing, you likely have a corrupt filesystem. At this point, you must be extremely careful not to write any further data to the drive, because doing so can cause you to lose existing data.
If the drive mounts, that means the hardware is likely to be operating correctly (though there could be a controller issue). Your drive may be in imminent failure due to bad sectors or something as yet unknown. The very first step is to verify whether the SMART status of the drive is still passing, or if any errors are being self-reported by the controller. If the latter is true, you are extremely hosed and need more professional service than what is available on this forum, because it means any further attempt to even power on the drive could result in data loss.
If SMART is passing, then your drive is likely still operational but you have some kind of data corruption. If the AV software is seeing the files but not the OS, then you could try creating a new user account and try mounting the disk from within that account, to eliminate the possibility the problem is due to your system settings. If this fails, then the next step is to boot from DVD as root user. Try reading the disk from there. If this fails, try running Disk Utility from the boot DVD.
If ALL of this fails, then the next step is to install a program like DiskWarrior, which is able to fix filesystem corruption beyond Mac OS's abilities with Disk Utility.
If DiskWarrior fails, then you are back at square one. You can try things like mounting to a Windows or Linux system, but it is unlikely to work. The last resort is to consult professional data recovery services, with typical rates in excess of $1000 per drive depending on the nature and severity of loss.
Hard drives are amazingly fragile things. It is a miracle of physics that they even work at all. To give you an rough idea of how modern hard drive technology has improved over the past 3 decades, drive capacity used to be measured in tens of megabytes. Now they are measured in terabytes, a full five orders of magnitude greater. In terms of data density, that is like comparing a punch card to a CD. That is how much more dense the data is packed on these modern drives.
They are also astonishingly cheap for what they are and how much critical data we put on them.
So here's my advice, if you ever get the data off your drive. (1) Don't buy Seagate external HDs, they're not very reliable. (2) Buy pairs of drives and RAID 0 them (mirror). It costs twice as much but you are much less likely to suffer simultaneous data loss. (3) Chuck old drives. 250 GB is nothing these days. Expect a typical HD's lifespan to be about 3-4 years, tops, even if you don't use it constantly. (4) Back up frequently, and back up critically important data--i.e., things you simply cannot lose under any circumstances--to multiple media (e.g., HD, DVD, CD, online servers, flash memory).
Good luck.
5D came later..
