I want to copy data from an old drive to a new drive. The laptop has a connection for only one drive. I see a number of splitters advertised for SATA drives, but have no idea what I need.
Use a USB to SATA cable, the SATA Multipliers and related are not well supported.
What you want is a SATA port multiplier:
In reading up for another question I found some discussion on port multipliers that indicates the success is highly dependent on the chipsets in play. In particular, the Backblaze Pod folks found that Silicon Image (SiI) chipsets SiI3132 and SiI3124 play well with the SiI3726-based port multipliers they were using, but that their motherboard's onboard Intel ICH10 ports did not.
Addonics makes several port multiplier products that works with those SiI chipsets:
You can also find SATA hubs like this one; it also appears to be based on the SiI3726 chipset:
You'll have to do some digging to find out exactly what SATA chipset your laptop uses; it's likely integrated into the system southbridge, so the Addonics products may not be appropriate. You can probably identify your chipset from the Windows Device Manager to be sure. If your research indicates your laptop's chipset will not play nicely with the SiI port multipliers, you're probably better off going with Chris S's USB suggestion.