The Z600, as some may know, has "pluggable" bays where a 3.5" HDD slots into a carrier frame then literally slides in and plugs straight in to a socket at the back and locks in place.
But using smaller SSDs (or ex-laptop HDD) you need to get creative as even with adapters nothing lines up. So I bypassed the power/SATA socket and ran cables directly. For power I used a Molex-SATA adapter cable as I have many many times before. The data cable I just ran straight from the motherboard. A bit untidy, but it worked fine - well in 2 out of 3 systems.....
Then, on a mission to tidy up (and fit second/third drives) I looked for a better solution and found Orico Hard Drive Enclosures, which directly convert smaller drives to standard 3.5" SATA HDD enabling use of the original carrier frames and combined power/data SATA sockets at the back.
Since doing that I've not (so far) had a single "hang" so my conclusion is that somehow the (very simple) Molex-SATA power adapter cable was not quite providing everything the SSD needed at boot time.