Problem has been introduced in Lucid on my AMD64 workstation. Prior to upgrade, both Free Agent drives work properly (though they would sleep and disconnect). 

Kernel: 2.6.32-12-generic

dmesg: 
[ 2442.200081] usb 1-3: new high speed USB device using ehci_hcd and address 13
[ 2457.320079] usb 1-3: device descriptor read/64, error -110
[ 2472.550343] usb 1-3: device descriptor read/64, error -110
[ 2472.781258] usb 1-3: new high speed USB device using ehci_hcd and address 14
[ 2487.900078] usb 1-3: device descriptor read/64, error -110
[ 2503.132586] usb 1-3: device descriptor read/64, error -110
[ 2503.362536] usb 1-3: new high speed USB device using ehci_hcd and address 15
[ 2508.390242] usb 1-3: device descriptor read/8, error -110
[ 2513.520107] usb 1-3: device descriptor read/8, error -110
[ 2513.750072] usb 1-3: new high speed USB device using ehci_hcd and address 16
[ 2518.780123] usb 1-3: device descriptor read/8, error -110
[ 2523.910123] usb 1-3: device descriptor read/8, error -110
[ 2524.022609] hub 1-0:1.0: unable to enumerate USB device on port 3
[ 2524.560371] usb 2-3: new full speed USB device using ohci_hcd and address 12
[ 2539.742553] usb 2-3: device descriptor read/64, error -110
[ 2555.030059] usb 2-3: device descriptor read/64, error -110
[ 2555.320070] usb 2-3: new full speed USB device using ohci_hcd and address 13
[ 2570.500081] usb 2-3: device descriptor read/64, error -110
[ 2585.790072] usb 2-3: device descriptor read/64, error -110
[ 2586.080058] usb 2-3: new full speed USB device using ohci_hcd and address 14
[ 2591.113111] usb 2-3: device descriptor read/8, error -110
[ 2596.241122] usb 2-3: device descriptor read/8, error -110
[ 2596.533004] usb 2-3: new full speed USB device using ohci_hcd and address 15
[ 2601.563110] usb 2-3: device descriptor read/8, error -110
[ 2606.690113] usb 2-3: device descriptor read/8, error -110
[ 2606.800289] hub 2-0:1.0: unable to enumerate USB device on port 3
[ 3173.220080] usb 1-4: new high speed USB device using ehci_hcd and address 17
[ 3188.342559] usb 1-4: device descriptor read/64, error -110
[ 3203.570045] usb 1-4: device descriptor read/64, error -110
[ 3203.800080] usb 1-4: new high speed USB device using ehci_hcd and address 18
[ 3218.920091] usb 1-4: device descriptor read/64, error -110
[ 3234.152295] usb 1-4: device descriptor read/64, error -110
[ 3234.380063] usb 1-4: new high speed USB device using ehci_hcd and address 19
[ 3239.410164] usb 1-4: device descriptor read/8, error -110
[ 3244.542664] usb 1-4: device descriptor read/8, error -110
[ 3244.770113] usb 1-4: new high speed USB device using ehci_hcd and address 20
[ 3249.802667] usb 1-4: device descriptor read/8, error -110
[ 3254.930421] usb 1-4: device descriptor read/8, error -110
[ 3255.042557] hub 1-0:1.0: unable to enumerate USB device on port 4
[ 3255.580104] usb 2-4: new full speed USB device using ohci_hcd and address 16
[ 3270.760076] usb 2-4: device descriptor read/64, error -110
[ 3286.050074] usb 2-4: device descriptor read/64, error -110
[ 3286.340211] usb 2-4: new full speed USB device using ohci_hcd and address 17
[ 3301.520451] usb 2-4: device descriptor read/64, error -110
[ 3316.810201] usb 2-4: device descriptor read/64, error -110
[ 3317.100077] usb 2-4: new full speed USB device using ohci_hcd and address 18
[ 3322.133592] usb 2-4: device descriptor read/8, error -110
[ 3327.260413] usb 2-4: device descriptor read/8, error -110
[ 3327.550134] usb 2-4: new full speed USB device using ohci_hcd and address 19
[ 3332.580425] usb 2-4: device descriptor read/8, error -110
[ 3337.710420] usb 2-4: device descriptor read/8, error -110
[ 3337.822558] hub 2-0:1.0: unable to enumerate USB device on port 4

lsusb: 
Bus 002 Device 003: ID 03f0:5411 Hewlett-Packard OfficeJet 4300
Bus 002 Device 002: ID 17ef:6009 Lenovo 
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 012: ID 054c:031f Sony Corp. 
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

What appears to be clear is that the problem is very much device specific. The USB 2.0 module appears to behave well with some devices - my printer, DVD-R drive and select Flash drive work - but very poorly with others like other Flash drives and the aforementioned Seagate external hard drives. 

And at least with my workstation - the problem appears to have been introduced with Lucid. 

Also, this may be a duplicate of Bug #: 54273.