[VBox-users] Newbie question: how can i dynamically (and/or after the fact) control the location of the .vdi files?

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When i got my laptop, in preparation for using vBox, (because i'm a vBox newbie)
i partitioned my hard disk like this by shrinking and creating partitions:

C:\  225 GB:  windows 8.1 O/S.   (factory installed)
E:\  100 GB:  intended for win10 .vdi files
F:\  100 GB:  intended for win2012r2 .vdi files
G:\  100 GB:  intended for win8.1 .vdi files

When i set up my first instance of Windows 10, i switched to "Expert Mode"
because i hope to find a way to control the placement of the .vdi file.

i failed and vBox 5.0.14 (latest stable version AFAIK)
placed the .vdi here without giving me a choice:

   C:\Users\<my user name>\VirtualBox VMs\10pro\10pro.vdi

MY NEWBIE QUESTIONS:

(a) what is the SAFE workflow to move the above .vdi file to my logical E: drive and tell vBox i've done that?

(b) is there a way to tell vBox in advance where i want it to place each .vdi that i create.

(c) if i take a snapshot of a .vdi, will it take as much hard disk space
    as the .vdi before the snapshot?  (i.e., if .vdi is 20gb, will .vdi plus
    snapshot be 40gb?)

Thank you/Gerry

P.S.:  Kudos to all of the developers who have made vBox awesome
       and kudos to Oracle for keeping vBox Open Source and free.