Can I replace the WD My Cloud 2TB HDD with a small SSD?

With regards to this NAS:
http://www.amazon.co.uk/WD-Cloud-Personal-Storage-NAS/dp/B00FOKN7FG

Can the internal 2 TB HDD be replaced with a 120 GB SSD drive? I don’t care about the space reduction. Will the NAS still work if I managed to clone the OS and partitions to a small SSD (assuming that it is partitioned and obviously reducing the size of the data partition)?

It will almost certainly work. You are replacing one SATA storage device with another SATA storage device.

The only tricky parts could be:

1. Does it actually use SATA internally (almost certain, but not mentioned on the page you linked to). Assuming it does use SATA:
2. Does it physically fit?
   E.g. do I need something to hold the 2½ SSD in place if I replace a 3½ inch disk?
3. Does it draw enough power. (PSU’s do have a minimum load.)
4. Does the NAS support TRIM? Note that life without trim is not a problem in itself. I am typing this from a trimless setup which has been in use since 2009. But if you fill the SSD in the NAS up to near 100% capacity and keep changing a lot of files then performance might degrade over time.

Cloning the OS/firmware to the new drive might not be necessary. You probably can plug in the new drive and then to a hard reset and have a clean start with the new disk.