If you like t420 try the t420s. Same laptop but slimmer and makes it much more transportable. Still has ultra bay, great keyboard, and easy access to everything. I have 3 around the house for various family members and it works great :)
In need of an quick alternative I bought a Dell XPS 13 9343. I like this a lot - it has a way nicer display, the form factor is just great for my needs, build quality is good and even the keyboard feels right (missing a trackpoint tho). Unfortunately it supports Microsoft's "Modern Standy", which is a fancy name for s2idle - which just doesn't work using Linux, at least I couldn't get it to work . When in s2idle it looses power and does not reliably wake up, in s3 it goes to sleep but needs a hard reset to wake up.
Waiting for my t470s now, as far as I know the latest Thinkpad generation stat supports classic s3 sleep.
*: Actually it lives on as a small NAS/Homeserver. It even packs three disks (M2 SSD, HDD in the HDD slot and an Ultrabay-HDD adpter).
I really miss the Ultrabay as well. I did the same thing, used to use it for a second HDD for extra storage.
Really though I miss the old old days when you could get an ultrabay battery. What do I need in a laptop more than storage? Battery life. I had a Fujitsu Lifebook which had an "ultrabay" (their version of it, at least) and that was the feature I used most for it.
It seemed to disappear all at once, I wonder if it was FAA regulations around flying with batteries that killed it or what.
Well, nowadays you can get a power bank with Power Delivery and charge your laptop via USB-C port
Moreover, that’s a relatively recent technology and there was a good decade (or more) between the point when bay batteries disappeared and usb-c charging became any sort of commonplace occurrence outside Apple products.
But I'm kind of glad, that it finally had an error. I wanted to buy a newer one for a long time, but I didn't want to toss out a perfectly functioning notebook and it was too beat-up to sell or even donate it.
Don't get me wrong; I'm proud of how far I stretched it (and it remains a respectable machine for what it is), but I just want to put this out there if anyone's thinking of trying to dump heavy modern web processing on the T420/T420s dinosaurs.