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Are you sure you provided the correct 'Host-ID'?
Note that 'Host-ID' is often defined as an ID number ties to the hardware of your PC; see here for a thread on mathcad 14 licensing, beware, it's old. In many cases that's the MAC address of your Network Interface (note that you may have two of those, the LAN and the Wi-Fi...)
I would assume that you need to provide the network MAC address (six two-digit hex numbers separated by colon, like A1:B2:C3:D4:E5:F6); this is generally required for a Flex-LM license.
The volume/serial number of a harddrive was used with the Cdilla licensing (Mathcad 2001i through Mathcad 11 at least) and generally consists of two 4-digit hex numbers separated by by a dash, like A1B2-C3D4).
If it really is the HD volume serial number that is required as Host-ID... you can change it to match what Mathcad appears to expect, and see if that helps.