i just realized these lenovo thinkcentre m900 models come in different form factors and your case (and the innards) may be different then shown in this video. The even slimmer case styles will be more limited on options and I'm trying to find that now
With that Lenovo Thinkcentre M900, I'd recommend opening it up and installing a larger drive internally.
If using MyNodeBTC, you will need 2 drives total as its based on separating OS from Data and permitting reinstalls of just the OS with ease. So in this case, 256 GB would be overkill for the OS drive (good, you'll probably never run out of space for your log files!) and you can add a larger internal for the Data drive.
Performance will be much better with an internal drive connected via SATA cables vs conversion through USB 3 interface being external.
This unit doesn't have M.2 suitable for storage so it won't support the smaller NVME sticks. An internal SSD drive will be fine. I've used low cost TeamGroup brand drives as well as Inland from Microcenter, but if you want brand name, the Samsungs are popular.
If you haven't seen it already, here's a good video on a tech explaining the process of working on that particular machine so you can get an idea where everything is https://www.youtube.com/watch?v=SM264QmyZaY
Discussion
Tiny model https://www.youtube.com/watch?v=ZnjVSvGMtSE
Shows an 2.5" drive bay good for an internal SSD.
There is a M.2 2280 slot on this model
Until you have it and open it, you wont know if the 256 GB drive that comes with it is an SATA (more likely) or M.2 type. Whatever it is, you'll want to get the larger drive for the other type