FreeAgent Pro - 500GB, 7200 RPM, USB 2.0, eSATA, External 500 GB Hard Drive: SIR i hav connected my frnds seagate 500gb externa...

Are all drivers properly installed, specially USB 3? Did you connect USB with power source?

WHat you could do is test other usb ports on the system or you will have to remove the drive from the caddy and install it as a slave inside your pc and see if you can access the data,
If you can then you know its the caddy at fault

Here is another possibility that i found on another forum

"I was playing around in Device Manager, unplugging and replugging the hard drive. Finally I found that although it wasn't listed under Disk Drives or Removable Storage
, it was listed under USB controllers. It had a little x on the icon. Upon right clicking it I found that it had somehow become disabled on my computer. I'm not sure how that happened. I know I didn't do it."
or you can try this
Try this fix for missing usb ports by JohnWill

Create a file with NOTEPAD containing the following lines and save it as FIX.REG
-------------------------- Use text after this line --------------------------------
REGEDIT4

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment]

"DEVMGR_SHOW_NONPRESENT_DEVICES"="1"
-------------------------- Use text before this line --------------------------------

Double click on FIX.REG and say yes to the Merge Into Registry question.

Unplug ALL USB devices.
Open Device Manager.
View, Show Hidden Devices.
Uninstall all devices under USB Controllers.
Uninstall all devices under Disk Drives that you know are not present.
Uninstall all devices under Storage Volumes. Say no to any reboot prompts until you are finished. Also, if a Storage Volume doesn't uninstall, ignore it and move to the next one.
If you have a yellow ? with unknown devices, uninstall all of the entries there as well.

When this is done, reboot TWICE.

Reconnect the USB devices and see if they're recognized properly.

NOTE: If you have a USB keyboard and/or mouse, you'll have to modify the instructions and leave enough parts for those to function.