You have 2 choices. One is easier than the other.
Before doing anything, make a backup of the database! (Database | Backup Database...)

Option 1:

First if you have top level folder, such as Digikits and the rest of the kits are in that folder this method is the easiest and it is the directions I give people that have failed EHD's.

1. Outside of ACDSee copy over all your digital kits to your new EHD
2. Go to Database | Database Maintenance.
3. Now it's going to process for a while while it is collecting stuff about your database. It says on the bottom... Please wait while ACDSee is collecting and summarizing Database information... You need to wait until this disappears before you can go on and this could take minutes.
4. in the Database Content: click on the + signs to expand out your EHD
5. Then you want to click on the top level folder on your old EHD,
   
6. Click on Change binding
   
7. Choose the top level folder on your new EHD.

In my video example here, Change Binding I demonstrate the above.
In my video example here is what I am doing:

• I have all my images residing on my EHD (EHD is called Digital Kits)
• I want to switch it to U (SimpleDrive) where I have copied all my digital kits.
• I have everything in a folder called "Digital Scrapbooking Library"
• So I click on the Digital KitsDigital Scrapbooking Library
• click on Change Binding and
• select U:\Digital Scrapbooking Library (SimpleDrive)

Option 2:

Is to copy/move all over the digital kits from one EHD to the new EHD with ACDSee. This works but a word of caution. Only copy over about 20 - 30 folders at a time. ACDSee doesn't seem to like copying over huge amount of folders at a time.   See this help:  Moving in ACDSee

If you copied the files over, you will have duplicates. To remove the duplicates:

1. Go to Database | Database Maintenance.
2. Select your OLD EHD or the top level folder containing your digital kits.  I don't know if it will allow you to select the whole EHD.
   
3. Choose remove dbInfo That will delete your duplicates