I’ve commented on attempting to use an SSD (two different SSD drives) within the My Cloud enclosure in another thread. Never could get it to work on a first gen single bay My Cloud. Seems there is something blocking the ability to use SSD drives internally. One suggestion at the time was to use a RS232 to pull boot information from the My Cloud to see what error it’s throwing at boot preventing the use of a SSD drive. I just reverted to using an old 2.5 inch mechanical laptop hard drive instead within the My Cloud enclosure.
[Guide] How to use a 2.5 inch hard drive in a single bay My Cloud
http://wdstagingsplit.staged-by-discourse.com/t/guide-how-to-use-a-2-5-inch-hard-drive-in-a-single-bay-my-cloud/256912
Note on SSD Drives: I tried using two different spare SSD drives (a 64GB and 120GB drive) and was not able to get either one to properly boot. Both got stuck with a Red front LED and no Dashboard access. Multiple attempts at cloning and manually running the unbrick process failed. Not sure if its an issue with the drive small size or with an incompatibility between the firmware/hardware of the My Cloud and using an SSD drive as the main drive. YMMV Others may have better luck with their SSD drives (or not).
