ssd detected but cannot be used in finder/disk utility

This is a classic case of a drive cable failure!

You'll need to follow this IFIXIT guide: MacBook Pro 13" Unibody Mid 2010 Hard Drive Cable Replacement and this is the part you should put in: MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable. Yes, this is a newer models part which is better than the original part.

Sadly you are likely going to need to reformat your SSD. You might want to try to save what you have on it by using a SATA to USB adapter like this one: Startech 2.5" SATA to USB adapter

The best way I can answer this is think it like water going though a pipe. If the pipe is leaking some water still gets though, just not all of the water you expected.

Thats the case here as the data pressure is the water so a low pressure (low rate of data) will get though the weak connection but as soon as you over pressurize the connection you start loosing too much.

We continually replace them in our systems. My users are hard on their gear banging the system around. The SATA cable flexes so over time the thin copper lines weaken. Often people will throw a HD back in and tell me its working now so how could it be the cable! This gets into the ability of the SSD being able to move more data across the connection. For reference:

• Samsung Spinpoint 2 TB HD SATA III (6.0 Gb/s)
  • Write - 250 MB/s (2.0 Gb/s)
  • Read - 300 MB/s (2.4 Gb/s)
• Samsung 850 EVO 2 TB SSD SATA III (6.0 Gb/s)
  • Write - 452.3 MB/s (3.62 Gb/s)
  • Read - 512.7 MB/s (4.1 Gb/s)

As you can see the SSD can move more data in or out Vs the HD