Multiple (related?) issues with external hard drive connectivity

Question

Apologies if this has been discussed but I have been unable to find a solution or explanation after searching here and on other forums.
I recently purchased an external hard drive (1TB Iomega Select Portable, bus powered, 2.5", USB 2.0). After formatting with two HFS+ partitions the device began disconnecting unexpectedly and afterwards was often not recognisable by my mac until I rebooted. I returned the product and had it replaced by another of the same model.
The replacement external hard drive worked fine at first but hours later began showing the same symptoms. _It now connects briefly then a few seconds later disconnects._ Occasionally it seems to mount and work fine, but the problem always comes back. In the times when it is working, _Time Machine recognises the drive but only backs up at a rate of about 1KB/s!_
_I don't know if this is a problem with the external hard drive, my USB ports, or settings on my mac._ Any advise you could offer would be greatly appreciated as I've run out of troubleshooting options with my limited knowledge of computers.
_**Some tests I've done:_
1) When it's not being recognised in Finder, it's also not recognised in either Disk Utility or System Profiler
2) At one stage after repeatedly disconnecting, two other external hard drives and two thumb drives that I know work fine normally also failed to mount, but strangely two wireless mouse USB receivers worked fine. Both thumb drives were formatted as FAT32, one of the external hard drives had one HFS+ and one NTFS partition, and the other was entirely NTFS. When they weren't recognised in OS X, the two FAT32 drives and the NTFS partitions of the external hard drives were all recognised in Windows 7 via VMWare Fusion. After restarting the computer all of them now work on both Windows 7 and Mac OS X.
3) I tried the external hard drive on another computer (a PC) and it was recognised in Vista's 'Disk Management' with both partitions marked as 'healthy', but it just called it 'Disk 1' rather than showing the names I'd given each partition.
4) When I first started transferring files onto the drive I often got 'error code -36', but found a solution that seemed to fix that by deleting the '._' files from the source folder through Terminal.
5) I've tried different USB cables, both USB ports on my mac, and also connecting with and without the secondary part of the 'y' cable that was supplied with it.
Thanks very much,
Andrew
Message was edited by: andrewcarter

Answer 1

I have the same problem with some 2.5 drives. My iMac on 10.5.8 sees the drive fine but my older 2007 Mac Book Pro 10.6.6 would not see it and now my new Mac Book Pro had problems seeing the drive from my old Mac Book Pro in that box. I had to use migration utility over the network that took 18 hours to copy my user over. I bought a new drive box and now the drive seems to work fine. I have ejected it a couple times and all seems well. I hope it continues to function properly.