I recently resurrected an old Thinkpad T61 from the dead, and brought it up to date with the addition of a snappy SSD. There's a few, uh, interesting snags that make this less than simple and they're mostly documented as footnotes on forum threads and reddit posts so I thought I'd put down a moderately more formal documentation. This is a moderately technical upgrade, since it requires a BIOS update. Swapping the drives is actually very easy, old business hardware has quick-swap drive cages.
The T61 I upgraded is one of the Nvidia MX140M GPU-equipped models, and before upgrades it had the stock 5400rpm 160GB magnetic drive. The CPU is an Intel T-8100 Core 2 Duo, which puts it as made in 2008, and it came with the original 2GB of RAM. The cause for upgrading was mostly that my parents needed a Real Laptop to do more extended word processing/file handling/emails on and phones, especially iPhones, just Aren't Great for that.
Step one is, of course, an SSD. Any SATA II or III 2.5" SSD will do, in my case this was a Western Digital Green, 240GB. The T61 came out pre-SATA III, so you won't see speeds greater than 3Gbps no matter what you do. That's plenty for our purposes, saturating SATA II will allow you to boot windows in under 30 seconds and run applications pretty quick. There is, unfortunately, a catch.
The SATA controller in a T61 is also locked to 1.5Gbps, which means that even with an SSD improvements are... small. A good quality modern magnetic drive can easily surpass that. Fortunately, the number of people who love old Thinkpads is really, really high, so there's an unofficial BIOS update that'll unlock full SATA II speeds.
So, installing the Middleton BIOS. These are Very Unofficial, but they seem to work reasonably well. Visit this wiki page, assuming it still exists, and download the BIOS file in the T61 row of the table, version 2.29-1.08. In the event that it /is/ down, I have a copy available here. Upgrading from here can go one of two ways now. If you're lucky, and still have a 32-bit version of Windows installed on the Thinkpad, there's an executable file WINUPTP.EXE that will do the BIOS upgrade from within windows. For anyone else, you'll need to create a bootable disk image of the included ISO. I do not know if the ISO actually works, because I lucked out and had Windows 7 32-bit installed on this thing.
(Note: There are actually two BIOS updates in the file, one includes the option to swap the Fn and Ctrl keys, if, uh, you want that.)
The tar.gz on my site should match sha256 signature:
Make sure that you have a charged battery and the charger plugged in, and run the update, either with the executable or by booting off the disk. If nothing goes horribly wrong, it'll complete the BIOS upgrade. Reboot to check that you haven't bricked the system. If it boots, great, you've got a working and unlocked T61.
Now, of course, to install the new SSD. If you've got a windows install you may want to make a recovery disk to reinstall on the new drive (I had a key lying around). If you're using a free OS, uh, good job, you get a gold star.
There's exactly one screw holding in the T61's drive cage, it's the one that looks out of place on the underside, on the same side as the optical drive bay. Unscrew that, lift the cover out of the way, and you can pull on the black tab to extract the drive cage. Remove the rubber buffers and screws holding in the old drive, and screw the new drive in. Put the rubber buffers back on, slide the drive into the bay and replace the drive bay cover. Insert your install media of choice, and you're more or less ready to go. Get your OS up and running however you like it.
With all these changes made, you can run a disk benchmark of your choice, I usually use
sudo hdparm -t /dev/sdx on linux, or CrystalDiskMark on windows. Both agree on "somewhere around 270MBps" for read speeds on the disk. I don't care about write speeds and I don't remember them, but they improved too.
A small upgrade like this gives the Thinkpad a lot of flexibility. It's not going to be as fast or power-efficient as a newer dual-core laptop, (although mine has a full-blown GPU that gives it a nice edge), and you could add an SSD to any cheap celeron netbook for similar results, but it's not a bad trade especially once you compare the excellent quality and durability of the T61 to cheap netbooks.