computer doesn't recognize external hard drive

1) Restart the iMac and then reconnect the External Hard Drive.

2) Open Disk Utility in your Utility Folder and if the External Hard Drive is there highlight it's icon and click the blue mount button.

3) Never just unplug an External Hard Drive until after you first Ejected it from OS X.

2) The hard drive icon does not show up in the Utility Folder so there is nothing to click on to bring it up.

Open the program Disk Utility (in the Utilities folder in the Applications folder) and then see if the drive is listed.

If Disk Utility can not 'see' the drive, you may be able to resurrect it in different ways.

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If you have access to DiskWarrior, try using it to repair that drive. Even though the OS and Disk Utility can not 'see' the drive, DiskWarrior may be able to, and if so, to effect repairs.

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If you have access to a PC (windows machine), try connecting the drive to it. If you can reformat it on that machine to a PC (windows) format, do so.

Then bring it back to the Mac and see if you can get Disk Utility to 'see' it. If it does, reformat it again as a Mac drive, setting your partitions again.

Note - this procedure will erase everything on the drive.

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I know this has been said, but always dismount an external drive (USB or firewire) before disconnecting it. Two ways to dismount an external drive -

• Drag its icon to the Trash. When the Trash icon changes to an eject symbol, drop the drive's icon into it.

• Select the drive's icon by clicking it once, then select Eject from the File menu.

I use a Mercury Elite-AL Pro external drive with my iMac, from OWC -

OWC Mercury Elite-AL Pro FireWire 800/400, eSATA + USB2 external Solutions up to 2.0TB - Plug & Play Quad Interface

- it's a 1.0TB model with quad interface (I needed the drive to have both FW400 and FW800 ports, so I could use it to transfer files from my old 2000 G4 to my new 2010 iMac).

The drive comes with all needed cables, and has its own external power supply.

After I had transferred files from my old machine, I reformatted the external drive and (later) partitioned it into three partitions. One holds a clone of my internal drive when it had OS X 10.6.8 on it; the second holds a clone of my current internal drive config, using OS X 10.7.2; and the thrid partition is used for incidental storage. The first two volumes are bootable; I can switch between my internal drive and them by using Startup Disk in System Preferences, or via Startup Manager (at machine boot, hold down the Option key as soon as the startup chimes sound).

I've run several different clones onto it, erasing in between; and even partitioned it after the first clone (when I realized I would need more than just the one original volume). Not a single problem with it.

I can't speak to the value of current models of Western Digital and Seagate; both of those are manufacturers of hard drives, but I have had no experience with either as a manufacturer of an external drive (adding enclosure and bridgeware). Both brands have been around a long time as bare hard drives, but, unfortunately, that is no longer a good indicator of quality.

Given that we tend to use up more hard drive space than we thought possible when we first start, I'm not sure there really is a thing as 'too much' hard drive space.

Typically, the difference in price between a 500GB and a 1.0TB drive (all else the same) is about 25%; and between a 500GB drive and a 2.0TB drive about 50%.

If your internal drive is in the 500GB to 1.0TB range, I'd go for a 1.0TB external drive.

And, I'd strongly recommend using firewire rather than USB. Among other things, firewire is much faster than USB, and, although not as fast a connection as an internal drive, is fully adequate for booting purposes (USB is just too slow for booting for other than emergenies).