Your SSD may be choking on deleted data.
Try Alt/Option booting and leave it sitting powered up but untouched for 20 minutes or more. It may be able to do a grabage collect and revive itself.
If it wakes up, then install TRIM Enabler or equivalent, and Install TRIM Enabler on an alternate System Drive and run Disk Utility (Repair Disk) against the SSD. This provides a very satisfying "Trim-ing unused blocks" message, and everything perks up.
Thanks for the suggestions! Alt/option booting doesn't revive the disk. I have to get it out, turn it back again and startup. I als did a firmware-update on a Windows machine.
Still not working. I'm almost ready to trow it away and buy a more decent (working) SSD for the MacPro. Any suggestions?
By the way, the ticket at Corsair support desk remains unanswered after 10 days. Sending Tweets doesn't work. What kind of company it this?
Corsair forum - there is a lot of activity there, user help, not as much Mac related but is a section.
They brought that one out in 2011.
Samsung 840 EVO are fine, popular and good price. I have 3 of them and another on the way.SSD: Samsung 840 128GB
Corsair Neutron Series GTX CSSD-N120GBGTXB-BK
These Corsair models cost more but also less variation in writes over time, fewer use of spare cells over time.
Samsung 840 EVO
SSD Endurance 500TB
Icy Dock $15
Drive sled adapter $20 from OWC