How can I add a second hard drive to my ThinkPad W520? I did not see an additional internal bay or slot the way some ThinkPads like the W700 or W701 series have which supports a second drive.
There is a drive tray or carrier - ThinkPad Serial ATA Hard Drive Bay Adapter III, which can be install instead of DVD drive. Is it the only built in option?
I know that there are connectors which appear to be different from common SATA. (6+7 pins instead of 7+15 pins). Any limitations on port speed?
There are several ways to add a second or third drive, including the use of USB 2.0, USB 3.0, Firewire, eSATA, eSATAp, across the wire with NAS, etc.
Most people prefer using the Ultrabay hard drive adapter and a 2.5" HDD or SSD. The correct ultrabay adapter is 12.7mm high (P/N 0A65623) and if one accidentally uses the 9.5 mm thick ultrabay tray used on a T420s (43N3412), there will be a gap. The 12.7mm drive tray can support the use of either 7mm or 9.5mm thick drives, however 7mm devices will need adapter rails to fit properly.
Another option would be to use an external HDD connected to either USB or the 1394 firewire connection. The industry seems to be moving toward USB. The W520 includes the following external ports:
Two USB 3.0 (blue), one Powered USB 2.0 (yellow), one USB 2.0/eSATA combo port