Installing Adobe Photoshop - disk full when it isn't?

Question

Here it is, I am running windows 7 ultimate from a 40gb ssd card (which is almost full) however I save all of my programs, files, pictures etc. onto my secondary 3TB drive which is almost empty.
Trying to install adobe photoshop elements 12 - says that the C: drive is full (which is true) but I'm telling it to load onto the D: (which is almost empty).
When I click OK - to install - it highlights the C: drive and won't allow me to click on the D: drive again.
Does photoshop have to run from the C: drive where all of the windows components are or something? I am waiting on a larger ssd which I ordered, but I would still prefer it to go onto the D:
Thanks for anything that would help.

Answer 1

yes - you could have plenty of space free but have it fragmented.
If this is the case and a file cannot be written contiguously then it gets fragmented into the spaces.
With an SSD - although there is no actual physical fragmentation, the file-system still believes there is, and still has to walk through extents to find all of the pieces of a file - i'm not sure how much of an impact that would make on an SSD but the write amplification of splitting up a file is probably more significant (i.e. if it has to mixed with half empty MLC write pages).
it's a difficult situation because TRIM wont help with this... the file-system may actually need to be defragmented - and you don't want to do that on the SSD because it causes alot of writes that will reduce it's life...
I think the best thing to do would be to find out just how fragmented the file-system space is, if it is significant then clone the drive to a HDD and defragment it on that... then restore it back to the SSD afterwards.
The only defragmenter/optimiser that should be used on OS X system volumes is iDefrag. (other defragmenters do not carry OS X specific file definitions... that said, it probably doesn't matter that much when the target is an SSD, the main thing of importance is the space and file fragmentation.