On my Mac (OS 10.6.8), I noticed my Fantom brand external hard disk was going to sleep while I was in the middle of a Time Machine backup. The backup would freeze, and a pinwheel of death would ensue.

The drive is green because it "features the latest advances in technology." That may mean it periodically becomes dormant to save power. This thread doesn't mention GreenDrives, so the problem may not be the brand's “greenness.”

Whatever the source of the problem, this little perl script will keep your disk alive by writing to it periodically; I appropriately named this file—to be run from the Terminal as the disk is plugged in—./keepalive.pl.

#!/usr/bin/perl -w
use strict;  
$| = 1;
my $disk = "/Volumes/Gandhi";  
my $hidden = ".keepalive";  
chdir($disk) or die "$disk not mounted\n";  
if(! -d $hidden) {  
    mkdir($hidden) or die "can't create hidden directory\n";
}
chdir($hidden) or die "can't occupy hidden directory\n";  
while(1) {  
    print '.';
    sleep(rand(30) + 5);
    open FILE, ">file.txt" or die $!;
    print FILE rand(10);
    close FILE;
}
1;  

For Windows users, I'm not sure if this phenomenon will happen, but if you want to use perl on Windows, give Strawberry perl a try. Linux users can run this perl script effortlessly.

Even if this isn't a true, model-wide flaw—I really can't prove that it is—I wouldn't recommend a Fantom GreenDrive. I've never had this problem with other drives (like a LaCie Rugged for example); I would just go with another drive and save yourself the trouble.