I downloaded the GOG Galaxy beta client to give it a try. In the settings, I changed the games and extras directory to my D drive, but somehow missed the "Install games to" setting and ended up preloading The Witcher 3 to my C drive, which is a small solid state drive. Is there any way I can safely move the downloaded content from one drive to another without having to redownload it all over?

Thanks!

No, not at the moment. I hope that will be implemented in the future. Remember that Galaxy is still beta after all.

EDIT - never mind

so does it mean if I install GOG under D drive, all game will install to D or no?
this is headache as my GOG installs under C drive but out of space!

No, you can simply change the installation and download directories in the GOG Galaxy settings.

I did this too. I created a new folder on my D: drive, moved all the Galaxy-downloaded games into that folder (same structure), then added them from the client. Some of them took a while to verify, but it worked. I also downloaded Witcher 3 on my laptop from a fast connection, took it home and moved the installation file onto my desktop (my laptop was a slide-show at low detail levels!), and imported it the same way. Again, took a while, but it works.

This. Not sure why people are saying no, you most certainly can with the scan feature.

I did exactly as you said. First, I closed the client, then moved the whole Witcher folder to my D drive (under GOG Games). However, when I launched GOG Galaxy, there wasn't much I could do. The Play button was of course greyed out and under Manage, nothing is clickable except Show Folder, which does lead me to the D drive, but I can't import a folder or verify. It even downloaded the day one patch! It just says "WAITING FOR RELEASE"

Did you hit the plus symbol up top of the client then scan?

ID88G, like BKGaming says, once you launch Galaxy, you need to click the + button on the left column where your installed game list goes. Then you navigate to the folder containing your games, select them, and it should import them. That's what I did at least, worked both times that I tried it.

After you add the game via "+", does it auto-patch and update? I´m just curious because i have to move the game after the download.
The Client was set to on C:\ on default... and there is no pause or stop button.

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WARNING
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Since today(27th May 2015), GoG galaxy seems to be destroying my installation by deleting my <junctions> and replacing them with real folder names (thereby becoming an empty directory). Not sure what's going on, but it seems to be related with issues that are going on with the new DLCs (#3 and #4) and Galaxy. I'd advise not using the method described below for now... (I switched to importing the folder from my SSD instead until they fix this issue).
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You can move your installation folder using a <junction> (if using Windows) by doing the following... (If you're good with computers, jump the the tl;dr section near the end of this post)

NOTE: In the example shown in this post, the original Directory for Witcher 3 is
"D:\Witcher 3"
and the destination folder is
"E:\Games\Witcher 3".

First copy the entire Witcher 3 current folder to the destination folder you wish to use... ex: from D:\Witcher3, right click on the folder, select <Copy>, and select the drive/folder where the new destination for the game will be (E:\Games\ using the example for this post), then right-click and select <Paste>.

Once the entire copy is complete, you need to rename the original folder to, say, D:\Witcher 3.bak (for example).

After the rename (or deletion, although I'd wait until you try it first to confirm the switch works), open a CMD window in admin mode by clicking on the <Start> button, then type "cmd" without the quotes. Above the typing area you'll see cmd.exe... right-click on it then select <Run as Administrator>.

In the newly opened DOS window (admin mode), type the following to create what's called a <Junction>... I'll use my example in the following command...

mklink /j "D:\Witcher 3" "E:\Games\Witcher 3" (Including the QUOTES), then press enter.

There you go, the game still thinks it's using the same folder as before, but the junction makes it so that the game is now played (and updated, patched, etc) from the E:\Games\Witcher 3" folder instead. Note that Galaxy will still think it's installed in the "D:\Witcher 3" folder, but everything it does will be done <through> the junction which points to "E:\Games\Witcher 3".

I use this on games that benefits from an SSD or games I play all the time (Diablo III, WoW, etc).

Let me know if you have any questions... I'll recap below with just the commands themselves..

*********************************************************************************
tl;dr
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My original folder: D:\Witcher 3.
My new location: E:\Games\Witcher 3

1) Copy the D:\Witcher 3 folder to E:\Games\ so that the destination folder becomes "E:\Games\Witcher 3".
2) rename the original folder from "D:\Witcher 3" to "D:\Witcher 3.bak"
3) open a CMD window in admin mode.
4) type in the CMD window: mklink /j "D:\Witcher 3" "E:\Games\Witcher 3" then press enter.
5) Done :-)

/cheers

P.S. I always install my games on my game drive (which isn't the SSD, just a good mechanical HD), and create a junction to the SSD while keeping the original full installation as a backup (I have enough space)... once I'm done playing the game, I just need to delete the SSD folder(to make space for other-new games), delete the Junction then rename the old installation from "D:\Witcher 3.bak" back to "D:\Witcher 3".
If I deleted the original folder instead of renaming it when I created the junction, I copy the folder from the SSD to the installation folder before removing it from the SSD. This way, all the patches that were installed are still there.

Let me give you a quick guilde how to move games without any linking.

1. Move the current installed and working (in galaxy) gamefolder to wherever you like.
(Galaxy should now tell you "install file corrupted" or something.)
2. Uninstall the game in galaxy
3. Choose "import Folder" in galaxy, choose the folder where you copied the game and watch galaxy check the files in the NEW location and your done.

I just did this and it worked. I installed to my C drive (128 GB capacity). I needed to move it to the D drive. I just dragged and dropped the GOG folder to the D drive. Then I deleted the GOG folder from the C drive. I then opened the Galaxy App and uninstalled Witcher 3.

I then went into the Manage tab on the Galaxy App and chose "import folder" and I opened the GOG folder in the D drive and then I chose the Witcher 3 folder and that was it.

I installed Witcher 3 again to the Witcher folder and Galaxy simply verified the folder's contents and that took place in the time it took me to write this reply.

Thanks for the help!