Finally, I installed DashBoard on a Windows 7 box (my home PC runs Windows 8.1) to see how well the TRIM feature does on my external (USB attached) Ultra Plus SSD.
I used the TRIMcheck utility (*) on my drive. Even after an hour wait, TRIMcheck could not detect any reaction to the trim command (supposedly) sent by Windows. But as soon as I performed a manual TRIM in DashBoard (which retrims the whole filesystem) TRIMcheck detected it as successful!
To sum up, TRIM commands (supposedly) sent by Windows are not received by my external SSD. However, TRIM commands sent by DashBoard are perfectly efficient.
Maybe this is because DashBoard operates at a lower level than Windows, but whatever the reason, DashBoard’s TRIM feature is very valuable when using a SanDisk SSD as an external drive. It is true with Windows 7, and I have no reason to think that it would not be the same with Windows 8.1.
DashBoard’s TRIM feature cannot do any harm, and it can help some users, so why not leaving it?
(*) Roughly speaking, TRIMcheck works by writing a file containing a data pattern, then deleting the file (to have Windows send a trim command) and then checking (at any time the user wants) that the data pattern has been erased (which means the drive controller got the hint). An SSD controller does not necessarily reacts quickly to TRIM commands, but in the case of my Ultra Plus, I have some confidence it does. A while ago, I did some tests with my SSD internally attached (via SATA) and it went as expected : TRIMcheck detected the data pattern removal a few seconds after the file deletion.