Many external HDD owners find the notion of a portable drive an exciting and useful prospect and indeed it is. However one has to consider that because such technology is portable and can be taken from place to place then there is scope for problems occurring because of its ability to be moved. As one might damage inadvertently a USB stick so to might one damage an external HDD.
And even without damaging the drive it will over time [http://www.leedsdatarecovery.co.uk/external-hard-drive-recovery/|Data Recovery From External Hard Drive] degrade just as any hard drive will, especially if it is subject to a lot of heavy duty usage. Noticeable signs that your drive is degrading include slower than normal load and write times as well as occasional drop outs which render the drive invisible to the operating system's directory tree.