More likely a usage problem
I have three external USB hard drives, two 2.5" and one 3.5" - all home built by putting a regular disk in an enclosure from the local computer store. All drives were PATA. The 2.5 drives (40 GB and 80 GB) were from notebooks I upgraded and the 3.5" was a new Seagate 300 GB. I've had no problem with any of them, these on a PC - I see no reason why a Mac should be any different.
However, friends have run into problems, almost invariably with 3.5" drives. In a couple of cases, the problems arose when the units were moved while powered on. Unlike 2.5" drives that are assumed to be used in a portable device, though moving them while spinning is not recommended, 3.5" drives are designed for desktops and other such (relatively) fixed machines. I suspect that they are much more senitive to being moved while powered on and this may be the cause of some of the problems.
Good practice would suggest that you should place the drive on a firm surface where it is to be used, plug in the USB cable and then switch the power on (note, most 2.5" drives are self powered from the USB port, so plugging the cable in will power them up). Whichever type you are using, when finished, use the system tray icon to stop the drive (or the Mac equivalent - I assume it has one) and wait for confirmation that it is safe to remove the hardware, then turn off the power before you move it. If you don't get the confrmation or the system says it can't be stopped at this time, leave it until you next power off the machine (which you do overnight to save the planet, don't you?!).
This routine has worked for me - your mileage may vary.
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