This might be of interest for those with too many PSD: a few years back, I tested various programs to compress my PSDs. The result of these test was that the most efficient way was to compress them in .rar format using WinRar.

This might no longer be the best option, but I still use it and get the filesize to 1/3 of the original size, which isn't too bad.

Just hoping this might be useful to someone else.