SONAR doesn't care about drive letters. However older versions of SONAR relied on short file names for plugins to build a unique id. This causes problems when the short file name changes or if the plugin is installed on a hard drive which doesn't support short file names.
See the following post that describes this issue. The short file name problem was resolved in SONAR X3 by changing the plugin GUID system. However projects saved from older versions still contain the short name based ID's.
 
If you look at the Class ID of the NEW version of Altiverb on your system you will probably see a difference in the numbers. Normally SONAR brings up a list of choices to remap the plugin when that happens so I'm not sure why you are not getting that prompt. If you post the new class ID of Altiverb it might clarify why that doesn't happen.
 
Anyway once you resave the project on the system from SONAR you shouldn't see this problem again because the ID's will be unique now irrespective of short names.

Noel Borthwick [Cakewalk]
Normally SONAR brings up a list of choices to remap the plugin when that happens

This definitely doesn't happen all the time and I wish it did, I've had to give up on a number of older projects because there didn't seem to be any way to fix up plugin references.

msorrels

Noel Borthwick [Cakewalk]
Normally SONAR brings up a list of choices to remap the plugin when that happens

This definitely doesn't happen all the time and I wish it did, I've had to give up on a number of older projects because there didn't seem to be any way to fix up plugin references.

I've NEVER seen that happen.

Noel Borthwick [Cakewalk]
SONAR doesn't care about drive letters. However older versions of SONAR relied on short file names for plugins to build a unique id. This causes problems when the short file name changes or if the plugin is installed on a hard drive which doesn't support short file names.
See the following post that describes this issue. The short file name problem was resolved in SONAR X3 by changing the plugin GUID system. However projects saved from older versions still contain the short name based ID's.
 
If you look at the Class ID of the NEW version of Altiverb on your system you will probably see a difference in the numbers. Normally SONAR brings up a list of choices to remap the plugin when that happens so I'm not sure why you are not getting that prompt. If you post the new class ID of Altiverb it might clarify why that doesn't happen.
 
Anyway once you resave the project on the system from SONAR you shouldn't see this problem again because the ID's will be unique now irrespective of short names.

Noel, you may be right. 
Let's review my steps to my solution (which might not be necessary):

1. My old project  was referred to "Altiverb.dll" on my old computer path "H:\Audio\Vst32\Altiverb 6.dll"
2. In my new computer, I installed Altiverb in default folder (c:\program files (x86)\cakewalk\vstplugins\Altiverb 6.dll)
3. My solution was create an Alias for C: in Virtual Subst as "H:", and then copy Altiverb 6.dll to a folder with the same old path "H:\Audio\Vst32"

After this the old project was opened without errors.
 
But to my surprise, I decided to do a test: I dismounted H: (removed the reference H: from Virtual Subst) and also I deleted Altiverb 6 .dll from \Audio\Vst32, that is, I got the same situation I was in when the error of this issue was described, ie,  Altiverb 6.dll only in c:\program files (x86)\cakewalk\vstplugins.
 
After this, I opened an old project from backup, to be sure it was untouched. And the old project was opened normally!!!
 
How is this possible?
 
Would if I had only created a folder C:\Audio\Vst32 and copied Altiverb 6.dll to there, I would not have had this problem?
 

Oh crap, that's right Noel. I now remember seeing the list of possible plugs to choose from.
 

When I said same place I meant same relative path

rogeriodec
Noel, you may be right. 
Let's review my steps to my solution (which might not be necessary):

1. My old project  was referred to "Altiverb.dll" on my old computer path "H:\Audio\Vst32\Altiverb 6.dll"
2. In my new computer, I installed Altiverb in default folder (c:\program files (x86)\cakewalk\vstplugins\Altiverb 6.dll)
3. My solution was create an Alias for C: in Virtual Subst as "H:", and then copy Altiverb 6.dll to a folder with the same old path "H:\Audio\Vst32"

After this the old project was opened without errors.
 
But to my surprise, I decided to do a test: I dismounted H: (removed the reference H: from Virtual Subst) and also I deleted Altiverb 6 .dll from \Audio\Vst32, that is, I got the same situation I was in when the error of this issue was described, ie,  Altiverb 6.dll only in c:\program files (x86)\cakewalk\vstplugins.
 
After this, I opened an old project from backup, to be sure it was untouched. And the old project was opened normally!!!
 
How is this possible?
 
Would if I had only created a folder C:\Audio\Vst32 and copied Altiverb 6.dll to there, I would not have had this problem?
 

Hard for me to explain exactly what happened without seeing your inventory but the symptoms are consistent with short file name problems that we solved. Plugin load substitution works like this:
 
SONAR builds a list of all available plugins in the scan folders when doing a scan and creates unique guids for them. In older versions of SONAR this guid was dependent on the unique ID of the plugin and its short file name.
 
When attempting to load a plugin in a project SONAR looks for the guid from the  project file in the inventory. If there is a direct match (which happens most of the time) it loads the plugin. If a match is not found it does a "wildcard match" which looks for entries with the same unique id but different short file names. If it finds a single entry it is automatically substituted. If it finds more than one you get a substitution dialog. If there is no match found at all you get a missing plugin dialog.
 
So if you got no match then either the ID itself changed or the plugin was not scanned for some reason in the scan path itself. Those are the only options. 
Any as I said before once you resave after you have made the plugin load, you should not encounter this issue again as long as you don't rename your plugins or replace them with updated versions with a different ID. Also this only applies to VST2 plugins. VST3 plugins already have a concept of a guid as the identifier.

My problem was solved when I set Sonar to run as administrator.   Altiverb loaded fine!